OBJECTIVE To investigate the effect of sinusoidal electricity magnetic fields (SEMFs) on the proliferation and differentiation of osteoblasts in vitro. METHODS Calvarial osteoblasts of newborn rats were isolated by enzyme digestion and randomly divided into 3 groups after subculture. Two groups of cells were exposed to 50 Hz 1.8 mT SEMFs for 30 min/d in parallel and vertical, respectively. Those without SEMFs exposure served as control. The cells were observed under the contrast phase microscope each day. After 48 h, cell proliferation was assayed by MTT method. The alkaline phasphatase (ALP) activities and calcium contents were measured after 3, 6, 9, and 12 days. The ALP positive colonies were histochemically stained after 10 days and the calcified nodules were stained by Alizarin Bordeaux after 12 days. Expressions of ALP, bone morphogenetic protein-2 (BMP-2) and Osterix (OSX) mRNA were measured at 0 h, 24 h, 48 h and 96 h. RESULTS The cells exposed to the SEMFs were arranged in spiral appearance after 3 days. Compared with control, SEMFs inhibited cell proliferation (P < 0.01 or P < 0.05), but enhanced the maturation and mineralization of the osteoblasts. The results showed that SEMFs improved ALP activities, promoted calcium contents, increased calcified nodulues numbers, boosted expressions of ALP, BMP-2 and OSX mRNA. SEMFs with magnetic lines of force in parallel has stronger activities than those in vertical. CONCLUSIONS The SEMFs at 1.8 mT and 50 Hz inhibit the proliferation of osteoblasts, but enhance the maturation and mineralization of osteoblasts.