The effects of season on conception, sire and sex of a calf, calving number, number of calves and region of feeding in Okinawa Prefecture on gestation period of cows were analyzed in Japanese Black cattle. Data used for the analysis were records on 5, 552 calves produced by artificial insemination and registered to the Okinawa Animal Breeding Association in 1982. Means, standard deviations and 95% confident intervals of the gestation period in days were 285.2±4.6; 285.0-285.4, 286.2±4.6; 286.0-286. 3, and 285.7±4.7; 285.6-285.8, in cows delivering female and male calves and in total of them, respectively. The linear regression coefficients of the gestation period for the number of calves and calving number were -5.2 and 0.8, respectively. Gestation periods for female calves were about one day shorter than those for male calves. The gestation periods of cows on both Miyako and le islands, where animals were fed in stalls, were longer than those in Ishigaki and other small islands where feeding included grassland tethering. The gestation periods for cattle inseminated in autumn were shorter than those inseminated in spring. The gestation periods for calves born in spring and summer were shorter than for autumn and winter. Percentages of variance components for the number of calves, the breeding bull and the calving number were estimated as 59.4%, 2.7% and 1% of the total variance of the gestation period. Heritabilities with standard error estimates as calculated as those from sire to female, male and either sex of calves were 0.293±0.080, 0.255±0.077 and 0.293±0.072, respectively.