Objective：To investigate the effects of different durations of 50 Hz 3.6 mT sinusoidal electromagnetic fields （SEMFs） on the proliferation and differentiation of human umbilical cord stem cells （HUCSCs） in vitro. Methods：The HUCSCs were isolated from the neonate umbilical cord and randomly divided into 6 groups after one passage. The treatment groups were under 50 Hz 3.6 mT SEMFs respectively for 0.5，1.0，1.5，2.0 h，and 2.5 h each day，while the control group underwent no SEMFs treatment. All groups were observed under the contrast phase microscope each day. The proliferation was tested by MTT. At 15 d and 17 d after the SEMFs treatment the calcified nodules were stained by alizarin red and vonkossa respectively. At 4 d and 6 d after the SEMFs treatment the Collagen-Ⅰand BMP-2 mRNA were tested by real time PCR. At 10，12，14 d，and 16 d after the SEMFs treatment the ALP activity was tested. Results：the SEMFs were arranged in spiral appearance 7~9 d after the treatment.The SEMFs promoted the proliferation of HUCSCs，except in 0.5 h group. The ALP activity in 0.5 h and 1.0 h groups significantly increased at 14 d and 16 d after the SEMFs treatment. The calcified nodules number in 0.5 h and 1.0 h groups significantly increased after the SEMFs treatment. The Collagen-Ⅰand BMP-2 mRNA expression in 0.5 h and 1.0 h groups significantly increased after the SEMFs treatment. Conclusion：1.0，1.5，2.0 h，and 2.5 h treatment of 50 Hz 3.6 mT SEMFs promoted the proliferation of HUCSC，0.5 h and 1.0 h treatment of 50 Hz 3.6 mT SEMFs significantly enhanced the osteogenesis differentiation of human umbilical cord.