The purpose of this study is to evaluate the expression ofIGF-I, considered as the mediator of action of estrogen, and IGF-IA and IGF-IB, alternative slicing form of IGF-I, using 17beta-estradiol in MC3T3-E1 cells. We observed the effect on type I collagen and osteopontin gene expression and DNA synthetic activity of MC3T3-E1 cells, added by estrogen, IGF-I and combination and the interaction on proliferation and differentiation of MC3T3-E1 cells. The results were as follows : RT-PCR experiment for observing time-dependant IGF-I gene expression patternshowed IGF-IA and IB gene expression in both of control and test group. In these IGF-IA gene expression was appeared predominantly. In control, IGF-I geneexpression level was maintained until 24hr and then decreased gradually. In test group, IGF-I gene expression level increased as time goes by. Experiment measuring DNA synthetic activity, as it is added by 17beta-estradiol, IGF-I and combination, showed that first day , there was the tendency of more increase of synthetic activity in all test group than control but no statical significance(P>0.05), and third day, there was more increase of DNA synthetic activity in 17beta-estradiol group and combination group and it was statically significant. (P<0.005) Experiment for observing type I collagen gene expression pattern showed more increase of expression in 17beta-estradiol group than control and no significant difference in IGF-I group and combination group. Experiment for observing osteopontin gene expression pattern showed no significant difference in control and test group. In conclusion, 17beta-estradiol in MC3T3-E1 cells increased IGF-I gene and DNA synthetic activity simultaneously, therefore it appeared that IGF-I is related to the action of estrogen. Combination treatment of IGF-I and 17beta-estradiol has effect on cell proliferation but this effect is lower than IGF-I or 17beta-estradiol alone. However, combination treatment has not great effect on type I collagen or osteopontin gene expression thus little effect of cell differentiation.