In order to evaluating genetic improvement in a selected population, variation resulted from environment and genetic should be dissociated. Because of positive genetic correlation between milk production and fat and protein yield, selection for milk production often resulted in an increase in fat and protein yields however response for fat percentage is negative. However positive genetic trends between 26 to 173 Kg in a year for milk production in Holstein Cattle are reported in some studies that milk yield had less importance in selection index, also negative values are reported. Based on previous researches, genetic improvement of milk yield up to 1 percent due to selection is possible and from theoretical view, an improvement about 2 percent is reported. With attention to performed selection in the herd and using progeny tested sperms from foreign countries that have suitable genetic potential, this study was accomplished in order to estimating genetic parameters of milk production trait and investigating genetic and environment trends during 1990 until 2001.