Abstract: Objective 　To study effect s of ext remely low f requency elect romagnetic field ( EMF) on apoptosis of tumor cells in vit ro. Methods 　Put human osteogenic sarcoma cells (MG63) 、breast cancer cells (MCF7) 、liver cancer cells ( Hep G2) in 962hole plates. These plates were exposed to elect romagnetic field (10mT、20mT、40mT、60mT) for stimulation twice per day lasting 1 hour for 3 days. The sham-exposure controls were correspondingly established. Inverted phase-contrast microscope was used to observe the cultured cells. MTT assay were applied to reveal viable cells with the OD value, and flow cytomet ry was used to detect the apoptosis result s with Annexin V-FITC and PI. Results 　We compared the elect romagnetic field groups with the normal groups by MTT assay af ter 72 hours. Analyzing the data in SPSS statistical sof tware, we found the OD value of elect romagnetic field group s were significantly less than the control groups ( P < 0. 05) . The rates of apoptosis cells by flow cytometry revealed EMF groups were higher than that in cont rol groups. The cont rast of two group s was significant . Conclusion 　Extremely low frequency electromagnetic field conspicuously rest rained the growth status of tumor cells, and some tumor cells could be induced to apoptosis by EMF.