Heat stress during warm season is considered as a major cause of ecumenical low fertility in animals. Heat stress in reproductive tracts, can damage the ovarian follicle development, steroidogenic capacity, oocyte quality and luteal progesterone secretion. These factors may increase early and late embryonic mortality. In dairy cows heat stress is the inability to dispose of the heat load, so that the animal capacity is not enough for opposition the stress. The present study was conducted to determine the effects of season of year (summer and winter) on plasma progesterone levels, pregnancy status, open days and insemination rate in three Isfahan dairy herds. For this purpose, ninety Holsteins dairy cows (50 ± 10 days in milk) with different parities (2, 3 and 4, n= 15 per parity) used in the summer (n= 45) and winter (n= 45). In each season, blood samples were obtained of five cows per parity. Blood sampling was done in day zero (AI time) and days 7, 10, 14 after artificial insemination. Progesterone concentration was determined in blood serum. The data of daily maximum, minimum and mean of temperature and humidity was obtained from the weather stations near to each farm and the temperature humidity indexes (THI) were calculated. Reproductive parameters such as conception rate, number of inseminations and open days were recorded in summer and winter seasons. Results showed progesterone levels (ng/ml) in pregnant cows at fourteen days after AI, during summer and winter was higher than non-pregnant cows (P 0.01). In addition, progesterone levels of pregnant cows ten day after AI in winter season was higher (P 0.01) than non-pregnant cows. Fourteen days after AI, non-pregnant cows progestrone in winter was higher than non-pregnant cows progenstrone in summer (P 0.01). Also, the winter progesterone in pregnant cows was higher than pregnant cows in the summer (P 0.01). Progesterone levels in day AI (zero), and days seven and ten after AI were not significantly affected by season. Pregnancy rates in cows that were inseminated in winter were higher than cows inseminated in summer and the differences was significant in all parities (P 0.01). Cows inseminated in winter had lower open days compared with cows inseminated in summer (P 0.01). Number of inseminations in winter was lower than summer and in some parity the differences was significant (P 0.01) or (P 0.05). Generally the results showed season had significant effect on reproductive performance and blood progesterone levels of Holstains dairy cows in Isfahan and the winter season had higher reproductive performance. Keywords: Season, THI, progesterone, Reproductive performance.