Per2 regulates other molecular and biochemical processes beyond their established role in the regulation of the mammalian circadian clock, herein we investigated the growth inhibiting potential of Per2 in human K562 leukemia cells and the underlying mechanisms. The results showed that over-expression of Per2 induced not only cell cycle arrest at G2/M phase but also an increase in apoptosis, which was confirmed by characteristic morphological changes, FCM and evident DNA fragmentation. Further experiments confirmed both up-regulation of P53 and down-regulation of CylinB1and C-myc. On the other hand, while P53 was found to be down-regulated. CylinB1 and C-myc were up-regulated. after Per2 knockdown. In leukemia mice, Per2 transfection was shown to suppress cellular proliferation and accelerate apoptosis of K562 cells. Moreover, fewer leukemia cells were found to have infiltrated into the livers and spleens of the mice from the Per2 transfected group as compared with those from the control group. In summary, Per2 displayed a significant anti-tumor effect through cell cycle arrest and apoptosis induction in K562 cells. These data further support the emerging role of the circadian clock in critical aspects of cancer development and thorough research is underway on the mechanism of Per2 in the leukemia.