This study was carried out ot examine the sex ratios of offspring in swine. The sex ratios were obtained on the basis of the data from 15,892 pigs produced by 1,728 litters of Landrace, Large Yorkshire, Hampshire and Duroc breeds at National Animal Breeding Institute from 1975 to 1980. The results obtained are summarized as follows: 1. The sex ratio of total offspring was 51.47% for male and significant at 5% level, and Landrace and Duroc breeds were 51.46% and 52.27% for male and significant at 5% level, respectively, but no significant difference between the breeds. 2. The sex ratio of offspring by years was 52.74% for male and significant at 5% level in 1979, but no significant difference for other years and between years. 3. The sex ratio of offspring by seasons were a little highly 51.65% and 51.25%, for male in spring and autumn, but no significant difference, respectively. 4. The sex ratio of offspring by parities were 52.65% and 55.81% for male in groups of the 3rd to 4th and the 5th to 6th parity, and significant difference at 5% and 1% level, respectively. 5. The sex ratio of offspring by groups of litter size was 52.65% for male in 1 to 6 heads group, and significant difference at 5% level, but no significant difference between groups of litter size.