The Monte Carlo simulation method was applied to postulate the genetic changes for the body weight at 6 and 12 months of age of Korean cattle(Han Wool. As assumptions for gene action, each animal had 64 loci with two alleles. `A` and `a` or `B` and `b`, with 32 pairs in each genome to allow epistasis between loci. The epistasis allowed between two loci. The genome was arbitrarily divided into 8 sections and the gene mode simulated was as follows; On the body weight at 6 months of age, an additive effect of `A/B` gene was observed in 4 sections among 8 sections, while that of `a/b` gene appeared in I section for the body weight. An interaction between additive by dominant effect was obtained in 2 sections, but them was no gene action in 1 section. On the body weight at 12 months of age, there were additive effects of `A/B` gene in 5 sections, dominant effect in 1 section, additive by dominant effect in 1 section and additive effect of `a/b` gene, on the body weight. The heritabilities of body weight at 6 and 12 months of age were 0.36 and 0.45, respectively, and gene mode actions were adopted to estimate genetic changes in body weight at 6 and 12 months of age in Korean cattle.