This study was carried out to investigate the effect of breed, parity, year, season of farrowing and age of sire on gestation length and sex ratio at birth in swine. The data were obtained from 831 litters of Duroc, Hampshire, Landrace and Yorkshire raised at the Swine Breeding Station, National Livestock Cooperatives Federation from 1983 to 1987. This results obtained are summarized as follows: 1. Gestation length was shortest (113.5±.022 days) in Hampshire and longest (115.6±0.16) in Yorkshire (p$lt;0.01). Sex ratio showed no significant differences among the breeds studied. 2. Gestation length was 114.5±0.19 days for the 1st parity, which was significantly different from 115.7±0.14 days for the 7th parity only in Landrace. 3. Gestation length was significantly different among years in Landrace (p$lt;0.01) and Yorkshire (p$lt;0.05). 4. Effect of season on gestation length was significant in Landrace and tended to be longer in spring and summer than others. 5. Gestation length appeared to be longer as the age of the sire increased in Yorkshire.