The history of Baekje Kingdom, one of the Three kingdoms, is divided into three periods to the change of sociopolitical center, including its capital as follows: Hanseong Period (18 BC ~ AD 475), Ungjin Period (AD 475~538), and Sabi Period (AD 538~660). Though the Hanseong Period of Baekje Kingdom covers more than two thirds of the whole history of Baekje Kingdom (493 years), history and archaeological culture of the Hanseong Period is still unclear and even ambiguous comparing to the Ungjin and Sabi periods. Most of all, it is because of quite limited historical records and archaeological data available. In addition, negative attitude of the Korean academic circles to the early records of Samguksaki(三國史記) has been a critical obstacle to the study of early history of the Three kingdoms, including the Hanseong Period of Baekje kingdom. Author, who has attempted to combine historical records and archaeological data in order to reconstruct the history and archaeological culture of the early Baekje, specifically the Hanseong Period, has held positive attitude to the early records of the Samguksaki as far as possible. He(Author) came to realize that comprehensive understanding of Mahan (馬韓) society, one of the Three Han (三韓) Society was more than essential in the study of Baekje. According to historical records and archaeological data, Mahan Society represented by Mojiguk(目支國) ruled by King Jin(辰王) has been located in the middle and/or southwestern parts of the Korean peninsula from the 3rd~2nd century BC through the end of the 5th century or early 6th century AD. Mahan already occupied central portion of the Korean Peninsula, including the Han River Valley when King Onjo(溫祖王) first set up the capital of Baekje Kingdom at Wiryeseong (慰 禮城) considered to be modern Jungrang~Songpa-gu area of Han River Valley. From the beginning of the Baekje history, there had been quite close interrelationships between Baekje and Mahan, and the interrelationships had lasted for around 500 years. In other words, it is impossible to attempt to understand and study Hanseong period of Baekje, without considering the historical and archaeological identity of Mahan. According to the Samguksaki, Baekje moved its capital three times during the Hanseong Period (18 BC ~ AD 475) within the Han River Valley as follows: Wiryeseong at Jungrang-gu area of the Han River (河北慰禮城, 18 ~ 5 BC), Wiryeseong at Songpa-gu area of the Han River(河南慰禮城, 5 BC ~ AD 371), Hansan at Iseongsan fortress site(Historical site No. 422, 漢山, AD 371~391), and Hanseong at Chungung-dong of Hanam city(漢城, AD 391~475). Before 1990s, archaeological data of the Hanseong Period was quite limited, and archaeological culture of Mahan was not well defined. Only a few burial and fortress sites were reported to be archaeological remains of the early Baekje, and a few settlement and jar burial sites were assumed to be those of Mahan without clear definition of the Mahan Culture. Since 1990s, fortunately, a number of new archaeological sites of Hanseong Baekje and Mahan have been reported and investigated. Thanks to the new discoveries, there has been significant progress in the study of early Baekje and Mahan. In particular, a number of excavations of Pungnap-dong Fortress site(Historical site NO. 11, 1996~2003), considered to be the Wiryeseong at south of the Han River, the second capital of the Hanseong Baekje, provided critical archaeological evidence in the study of Hanseong Period of Baekje. Since the end of the 1990s, a number of sites have been reported in Gyeonggi, Chungcheong, and Jeolla provinces, as well. From these sites, archaeological features and artifacts representing distinctive cultural tradition of Mahan have been identified such as unstamped fortresses, pit houses cut into the rock, houses with lifted floor(掘立柱 건물), and potteries decorated with toothed wheel and bird's footprint designs. These cultural traditions reflected in the archaeological remains played a critical role to define and understand archaeological identity of the Mahan society. Moreover, archaeological data from these new sites reported in the middle and southwestern parts of the Korean Peninsular made it possible to postulate a hypothesis that the history of Mahan could be divided into three periods to the change of its sociopolitical center in relation with the Baekje Kingdom's political Situation as follows: Cheonan (天安) Period, Iksan(益山) Period, and Naju(羅州) Period. The change of Mahan's sociopolitical center is closely related to the sociopolitical expansion of the Hanseong Baekje.