Medicinal plants are known for their many advantages, including the ability to treat diseases such as cancer. Nigella sativa and its active constituent thymoquinone (TQ) have long been used in traditional medicine for treating various conditions related to the respiratory and gastrointestinal systems as well as breast, colorectal, gastric, hepatic, pancreatic cancers and leukemia. TQ has been documented to possess chemo-preventive and chemotherapeutic antitumor effects. Studies reported that TQ inhibits the growth of cancer cells in animal models and culture tumors. This review summarizes the in vitro and in vivo possible mechanisms of TQ anticancer effect.