Data from 285 Hanwoo(Korean cattle) steer calves was used to study the effects of the body weight and age on carcass traits. The steers were castrated at 4-5 months of age and slaughtered at 24 months on the average. The body weight at slaughter affected backfat thickness, longissmus muscle area, yield grade index and auction price of Korean cattle steer significantly(P$lt;0.05), but marbling score was not affected. On the other hand age at slaughter did not make any significant effects on backfat thickness, longissmus muscle area, yield grade index and auction price. Marbling score, however, was significantly(P$lt;0.01) affected by age. Age was more important factor than body weight to improve marbling score in longissmus muscle area. The optimum age for gene expression of the score in Hanwoo(Korean cattle) steer was estimated to be about 24 months of age.