This study was conducted to determine the main effects of breed, farrowing year, season, parity and generation on gestation length, litter size at birth, and pig weight at birth, 21 days and weaning in swine. The least square means were estimated on the basis of the data obtained from 1,728 litters of Landrace, Large Yorkshire, Hampshire., and Duroc breeds raised at National Animal Breeding Institute from 1975 to 1980. The results are summarized as follows. 1. Large Yorkshire and Duroc were superior in Utter size at birth and Landrace was heavier in the preweaing weight than in the other breeds. 2. The effect of breed for the little size at birth and pig weight at birth, 21 days and weaning was highly significant at 1% level. 3. The effect of farrowing year on the litter size and pig weight at birth was significant at 5% level and there was highly significant difference at 1% level for pig weight at 21 days and weaning. 4. The effect of farrowing season was highly significant at 1% level for pig weight at 21 days and weaning. 5. The effect of parity was highly significant at 1% level for all traits studied except gestation length. The litter size at birth was superior in the groups of the 5th to 6th parity than in the other groups and pig weight at weaning was heavier in the groups of the 2nd to 4th parity than in the other groups. 6. The effect of generation was significant at 5% level for pig weight at birth and was highly significant at 1% level for pig weight at 21 days and weaning.