Collected data in Animal Breeding Center of Iran from 1983 to 2003 was used to estimate the genetic parameters of milk yield in Iranian Holstein cattle. Genetic parameters were estimated, using single and three trait animal models. Records of three lactations were divided into thre esub groups according to average milk yield, climitical regions and herd sizes. All variance components in the three lactations for dry desert region were the highest and for the humid region the lowest. Heritabilities and variance components increased in three lactations for three herd sizes, by increase in the herd size. In the three milk yields, within the first lactation, additive variance was the highest for average milk yield and residual variance the highest for high milk yield, but in the high milk yield (within the second and third lactations), all variance components were in their highest amounts. In the first lactation, heritability for the high milk yield was the lowest and for the average milk yield the highest, but in the high milk yield within the second and third lactations, heritability was in its highest amount. The results of single trait and three trait analyses showed that there was not a considerable difference among the estimates of three trait analysis and single ones. Genetic and phenotypic correlations among the subsequent lactation periods were higher than those in the far apart ones.