The study was carried out to investigate the changes of hormone levels in blood serum during the estrous cycle and the accuracy of pregnancy diagnosis by estimation of plasma progesterone at 20 to 23 days after breeding in sows. The progesterone and estradiol-17 R from blood serum samples were assayed by radioimmunoassay method. The results of this study were summarized as follows: I.The progesterone concentrations in blood serum during the estrous cycle began to decline rapidly at 2 days before estrus, and thereafter decreased to 0.7±0.18ng/ml at on the day of estrus, and thereafter reached a peak mean level of 13.1±2.15ng/ml at 15 days after estrus. 2.The estradiol-17β concentrations in blood serum during the estrous cycle showed a peak mean level of 14.6±3.24pg/ml at 18 days after estrus, and decreased from 9.8 pg/ml to 14.2 pg/ml at luteal phase. 3.During 20 to 23 days after artificial insemination, the accuracy of pregnancy diagnosis from the blood serum progesterone levels were 96.4% for non-pregnant sows($lt;2.7ng/ml), and 86.1% for pregnant sows( 3.6ng/ml). The average overall accuracy of pregnancy prediction for non-pregnant and pregnant sows was 91.3%.