Cancer has become the leading cause of mortality since 2010 in China. Despite the remarkable advances in cancer therapy, a low survival rate is still a burden to the society. The antineoplastic activity of aqueous extracts of Cordyceps kyushuensis Kob (AECK) was measured in this study. Results showed that AECK can significantly inhibit the proliferation and viability of U937 and K562 when treated with different concentrations of AECK, and the IC50 values of U937 and K562 were 31.23 μg/ml and 62.5 μg/ml, respectively. Hoechst 33258 staining showed that AECK could cause cell shrinkage, chromatin, condensation, and cytoplasmic blebbing, and DNA ladder experiment revealed the evident feature of DNA fragmentation which showed that AECK could induce cell apoptosis. Moreover, AECK gave rise to intrinsic apoptosis through increasing the amount of Ca2+ and downregulating the expression of Bcl-2. Meanwhile, the level of Fas death receptor was elevated which indicated that AECK could lead to exogenous apoptosis in U937. The expressions of oncogene c-Myc and c-Fos were suppressed which manifested that AECK could negatively regulate the growth, proliferation, and tumorigenesis of U937 cells. This research presented the primary antitumor activity of AECK which would contribute to the widely use of Cordyceps kyushuensis Kob as a functional food and medicine.