Abstract: ObjectiveTo investigate apoptosis in mouse leukemia cell (WEHI-3) induced by Thapsigargin and its mechanism. Methods Apoptosis induced by Thapsigargin was examined by flow cytometry and Agarose gel electrophoresis of DNA and TUNEL staining.Free calcium（［Ca2+］i）was determined by Fura-2 fluorescein load techinique.RT-PCR was used to analyze the mRNA of Caspase-12. Results After exposure to Thapsigargin of 0.5,1 and 2 μmol/L,WEHI-3 cells were induced to apoptosis.The apoptotic index was examined by Annexin-V/PI double-stained，Agarose gel electrophoresis of DNA and TUNEL. The results showed that the apoptotic index of the experimental groups was significantly higher than that of the control group(For example:the apoptotic index of the experimental groups determined by Annexin-V/PI double-stained were (25.3±3.2)%,(46.7±3.9)% and (70.2±2.3)%, respectively, which were significantly higher than the apoptotic index of the control group which was (7.6±0.4)% (F=26.52，P＜0.01）;and the apoptotic index was in dose dependent manner.After exposure to Thapsigargin,that ［Ca2+］i in the experimental group was significantly higher than that of the control group［(156.5±10.3）nmol/L，(180.3±15.6) nmol/L and (10.7±15.3)nmol/L vs. (78.3±11.2）nmol/L（F=21.26,P＜0.01）］.The mRNA expression of Caspase-12 increased with an increase in Thapsigargin concentration. Conclusion Thapsigargin can induce the WEHI-3 to apoptosis by Endoplasmic reticulum stress of ［Ca2+］i overload,Ca2+ may play a promising target for cancer therapy.