Objective    To observe the effect of different magnitude and action time of static magnetic field on proliferation and surface ultra-structure in cultured osteoblasts.      Methods    Cultured osteoblasts were loaded onto the static magnetic field at the magnitudes of 0

8，50 or 160 for 24，48 or 72 h. Proliferation of osteoblasts was detected by MTT assay. Changes in morphology and surface ultra-structure of osteoblasts were observed under an inverted phase contrast microscope and a scanning electronic microscope

respectively.     Results    The proliferation of osteoblasts cultured under static magnetic field at the magnitudes of 8，50 and 160 for 24，48 and 72 h increased differently. No difference was observed in morphology

accounts and distribution of osteoblasts. However

the surface of osteoblasts became uneven or irregular

and the number of microvilli increased and became thicker and larger with fused vesiculation and blebs when the static magnetic field loading time was prolonged. Furthermore

the effect of static magnetic field at the magnitudes of 8 and 50 mT was significant on the surface ultra-structure of osteoblasts.     Conclusion    The proliferation of in vitro osteoblasts increases

and the surface ultra-structure of osteoblasts is differently rebuilt when the static magnetic field loading time is prolonged at the magnitudes of 8

50 and 160 mT.