This study was carried out to investigate the factors affecting milk traits, and estimate the genetic parameters of milk yielk, fat yeild, protein yield, fat percent, and protein percent. Besides, the genetic trends were estimated for milk production traits based on estimated breeding values. Records of Holstein dairy cows during 1991-2011 were used from Vahdat Industrial Agriculturists and Dairymen Cooperative of Isfahan. All records have been prepared and edited and then normalization of data for all traits and normal distribution of the residuals were evaluated. The pedigree file was produced according to CFC software. Best linear unbiased prediction was used to predict the breeding values, (co)variance components and genetic parameters based on the single and multiple trait approaches. The heritabilities of milk production, protein and fat were 27.1%, 24.52%, and 21.74%, respectively. Genotype and phenotypic correlations were estimated using restricted maximum likelihood and average of the two-trait animal models algorithm via AI-REML method. The genetic correlation for milk and fat was 0.93, for milk and protein 0.96, and for fat and protein 0.82. Animal breeding values ??versus the year of the birth was showed that the annual genetic trend for milk, fat and protein during 1991- 2011 were 31.81 ± 0.96, 1.04 ± 0.05 and 0.97 ± 0.04 kg, respectively. According to the results, it seems that genetic variation of the above traits in Isfahan Holstein dairy cows had suitable trend and the breeding schemes were successfully incresed.
