The study was conducted to obtain some genetic information for developing superior broiler breeder stock. Genetic variations were estimated with 997 progenies of $quot;4 × 4$quot; dialled crosses. The progenies were raised at the Livestock Experimental Farm, College of Agriculture, Seoul National University from March, 1974 to June, 1975. Body weights at 8, 24 and 64 weeks of age were investigated and analyzed into additive gene effects, dominance effects and heritabilities with Hayman`s model. The estimates of these effects were made with $quot;4 × 4$quot; dialled tables of three White Plymouth lock lines (RF, SF, TF) and a line of White Cornish (TM), and again with $quot;3 × 3$quot; diallel tables of three White Plymouth Rock lines in order to compare the effects from breed-crosses and line-crosses of the same breed. In body weights at the ages of 8 and 24 weeks, the genetic variation due to additive among parental lines showed significant differences in both cases of breed-crosses and line-crosses. However, dominance effects appeared in case of breed-crosses, and the effects of overall dominance and line dominance slowed significant differences. No significant differences were observed in genetic variation on body weight at the age of 64 weeks. Heritability estimates for the body weights at the age of 8, 24 and 64 weeks were 0.77, 0.74 and 0.14 in the White Plymouth Rock ; and 0.49, 0.35 and 0.08 in the crossbred of White Plymouth Flock and White Cornish, respectively.