Some researchers reported that beef marbling was improved by the supplementation of organic zinc to a diet satisfying zinc requirement. We studied the relationship between serum zinc concentration and marbling score or serum adipogenic activity in 40 fattened steers. To determine serum adipogenic activities of the steers, preadipocytes were cultured in medium containing the serum samples during differentiation. Although serum zinc concentration was not related to beef marbling score, it was positively correlated to adipogenic activity. Then, we studied the effect of zinc on adipocyte differentiation. Zinc was added into the medium with the similar methods except the addition of fattened calf serum. The activity of glycerophosphate dehydrogenase, a marker of adipocyte differentiation, was significantly increased by the addition of zinc in culture with or without insulin. These results suggest that zinc possibly improved beef marbling through increasing adipogenic activity during fattening.