This study was carried out to present the informations for the optimum age of selection based on partial records to improve egg weight in layers. The data used in this study were the records on 787 White Leghorn hens from two different strains, A and K, which were raised at College of Agriculture, Seoul National University since 1976. The traits analysed in this study were mean egg weights in each period divided into 4 week intervals from first egg and annual mean egg weight in lines A and K. Heritability estimates for mean egg weight in each period from sire component and dam component were; different in appearance in two lines. In line A, the fact that heritability estimates from sire component was higher than those from dam component except for EW 22 suggested the importance of the sex-linkage effect on the inheritance of the traits studied. But the result of line K was shown to be reverse. The heritability estimate for EW 30 that is mean egg weight from 29 weeks to 32 weeks of age was higher than any other periods in line A. In the genetic correlation between. annual mean egg weight and mean egg weights in each period, the coefficient of EW 34 was 0.999 for line A and 0.900 for line K. The figures were highest among all genetic correlation coefficients. Results obtained above suggested that it might be possible to select hens at the period of 33-36 weeks of age to improve egg weight based on early part of the record.