Nigella sativa (N. sativa, family Ranunculaceae) is a medicinal plant that has been widely used for centuries throughout the world as a natural remedy. A wide range of chemical compounds found in N. sativa expresses its vast therapeutic effects. Thymoquinone (TQ) is the main component (up to 50%) in the essential oil of N. sativa. Also, pinene (up to 15%), p-cymene (40%), thymohydroquinone (THQ), thymol (THY), and dithymoquinone (DTQ) are other pharmacologically active compounds of its oil. Other terpenoid compounds, such as carvacrol, carvone, 4-terpineol, limonenes, and citronellol, are also found in small quantities in its oil. The main pharmacological characteristics of this plant are immune system stimulatory, anti-inflammatory, hypotensive, hepatoprotective, antioxidant, anti-cancer, hypoglycemic, anti-tussive, milk production, uricosuric, choleretic, anti-fertility, and spasmolytic properties. In this regard, we have searched the scientific databases PubMed, Web of Science, and Google Scholar with keywords of N. sativa, anti-cancer, apoptotic effect, antitumor, antioxidant, and malignancy over the period from 2000 to 2017. The effectiveness of N. sativa against cancer in the blood system, kidneys, lungs, prostate, liver, and breast and on many malignant cell lines has been shown in many studies, but the molecular mechanisms behind that anti-cancer role are still not clearly understood. From among the many effects of N. sativa, including its anti-proliferative effect, cell cycle arrest, apoptosis induction, ROS generation, anti-metastasis/anti-angiogenesis effects, Akt pathway control, modulation of multiple molecular targets, including p53, p73, STAT-3, PTEN, and $PPAR-{\gamma}$, and activation of caspases, the main suggestive anti-cancer mechanisms of N. sativa are its free radical scavenger activity and the preservation of various anti-oxidant enzyme activities, such as glutathione peroxidase, catalase, and glutathione-S-transferase. In this review, we highlight the molecular mechanisms of apoptosis and the anti-cancer effects of N. sativa, with a focus on its molecular targets in apoptosis pathways.