Brown algae is variety of biological compounds, including xanthophyll, pigments, fucoidans, phycocolloids, and phlorotannins. Several studies concerning these types of compounds have pointed out the variety of biological benefits associated with the algae, including antioxidant, anticoagulant, antihypertension, antibacterial, and antitumor activities. Diphlorethohydroxy- carmalol (DPHC) is a phlorotannin compound isolated from the brown algae Ishige okamurae, with various biological activities in vitro and in vivo. Numerous studies have shown that antioxidant assist inhibition of accumulation of fat. So we studied that effect of DPHC isolated from Ishige okamurae modified the accumulation of fat on preadipocyte, 3T3-L1 cells in vitro. First, the viability of cell was analyzed after 0.4, 2, 10, 50 μg/ml of DPHC treatment using MTT (3-[4,5-dimethylthiazo-2-yl]-2,5-diphenyl tetrazolium bromide) assay. Second, proliferation of cell was analyzed after 0.4, 2, 10, 50 μg/ml of DPHC treatment through measure doubling time. 3T3-L1 cell differentiation into adipocyte was analyzed after induction in the induction medium containing DPHC. The metabolic activity was suppressed by DPHC in concentration dependent manner. Doubling of 3T3-L1 was delayed by the treatment of DPHC in concentration dependent manner. DPHC also inhibit accumulation of triglyceride in the adipocyte. The expression of the marker genes for adipocyte differentiation coincided with cytochemical results. Base on them, it is suggested that DPHC suppress proliferation of adipose precursor cell and differentiation into adipocytes.