This study was conducted to estimate the genetic variance components of the milk production and fat percentage of dairy cattle under heat stress conditions in Isfahan province. The data was collected from 34 dairy cow herds by the VAHDAT cooperative union and General Directorate of State Meteorological in Isfahan from 1990 to 2013. After editing and preparing the record and computing the Temperature-Humidity Index (THI), 341774 records of 37025 cows for milk production and 262956 records of 26187cows for fat percentage on the first lactation were studied. Dairy milk production records ranged from 5 kg to 60 kg for milk and milk fat percentage from 1 to 7%. Significantly of the effects on the production traits were firstly evaluated. Genetic components were estimated using random regression methods using software DMU. The results indicated that with increasing temperature humidity index phenotypic traits decreased. The decrease of the threshold THI=72 for milk production traits and for trait fat percentage of THI= 64 occurred. The mean heritability in different the temperature-humidity index in the study of for milk production traits 0/32 and 0/24 for fat percentage were estimated. The results indicated that with increasing temperature humidity index in 72 the genetic variance for both traits were increased. In on other words, the cows after heat stress (THI=72). Genetic differences were significantly increased. Result showed that genetic selection of animals to improve genetic resistance for heat stress can be considered in the studied herds. The results of this study indicate that the additive genetic variances were higher in early lactation (5 to 100 days of lactation), for both milk yield and fat percentage. This suggests that the effect of heat stress on cows in early lactation are greater than in the mid and late lactation than cows. Keywords : Heat stress, Milk Production traits, Genetic parameters, random regression, Holstein cows
