1. Objectives : The Crown Prince Sohyen(1612-1645) has been believed ro have been poisoned to death because of the political conflict with the King Injo. However, the Crown Prince Sohyen who was So-yangin(SY type) didn't be treated properly at that time. This paper is supposed to explain the reason why he died by reference. 2. Methods : The prescription which was made out to the Crown Prince Sohyen is analyzed, based on the Sasang Constitutional Medicine(SCM) through Shimyang-ilgy and Ulyudongkung-ilgy. 3. Results : 1) In the case of the prescription where So-yang medical stuffs were contained a lot or Gunyak(the main madical stuff) was So-yang one, the Crown Prince Sohyen was better, while that of Tae-eum and So-eum medical stuffs didn't work. Therefore, the Crown Prince Sohyen can be considered So-yangin. 2) The So-yang in's symptom was misunderstood as Hakjil and treated, which could lead him to be put to death 4. Conclusions : The Crown Prince Sohyen is supposed to be Soyang-in(SY type), and accordingly the So-yang in's symptom was misunderstood as Hakjil and treated, which could cause him to pass away.