The current study was conducted to investigate effects of sera taken from Hanwoo at different age on adipocyte differentiation. Sera were taken from Korean native (Hanwoo) steers at 6, 12, 18, and 24 months of age, respectively and supplemented to 3T3-L1 preadipocytes after the cells reached confluence and maintained 10 days thereafter. For the first 2 days (48 h), cells were induced to differentiate by addition of differentiation factors, methylisobutylxanthine, daxamethasone, and insulin. After the differentiation, the cells were incubated without differentiation factors except insulin. The cells lost their fibroblastic shape and showed round-up appearances after 10 days incubation with FBS and the sera of Hanwoo steers. Big lipid droplets appeared in the cells cultured with FBS and the sera taken from Hanwoo at 18 and 24 months of age. After 18 months of age, GPDH activity was statistically higher than 6 and/or 12 months of age (P < 0.05). Based on morphology and Sn-glycerol-3-phosphate dehydrogenase activities, Hanwoo steers expressed aged-dependent adipogenic activities, indicating that aged sera may result in high adipocyte differentiation. It is concluded that the 18 months of age may be ‘threshold’ to express major adipogenic activities. This may strongly support previous field studies reporting considerable increase in fat contents of Hanwoo carcass at over 18 months of age.