The biological activity of tissue specific stem cell is under the control of their specific microenvironment and the exogenous chemicals derived from digestive tract can be one of the constructing factors of that. It is suggested that the extract of brown algae Ishige okamurae has antioxidant-, apoptosis induction-, and antiinflammatory- effects. On the other hand, a few studies have shown that antioxidant assist inhibition of accumulation of fat. So we studied the effect of the extract of I. okamura on the cellular activity and differentiation of 3T3-L1 preadipocyte to adipose cell. The viability of cell was analyzed using 3-[4,5-dimethylthiazo-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) assay. Adipogenesis of 3T3-L1 cell was analyzed after induction in the induction medium containing the I. okamurae extract. The cellular activity was high compared with the vehicle and 0.05 mM caffeine in all groups of I. okamurae extract treated cells. The extract of I. okamura inhibited accumulation of lipids in 10 and 50 μg/ml. The expression of the marker genes for adipocyte differentiation coincided with cytochemical results. These results suggest that the extract of I. okamurae increases the cellular viability of adipose precursor cells. On the other hand, it suppresses the differentiation of preadipocyte to adipocyte and accumulation of lipids in concentration-dependent manners. It may be possible that the major component of the extract can be applied in the control of adipose tissuegenesis.