Genetic improvement of farm animals is the process of selecting animals of higher genetic merit than average to be parents of the next generation such that the average genetic merit of their progeny will be higher than the average of the parental generation. In practical dairy cow breeding programmes, many traits such as milk production and fitness traits (consisting of health, fertility, calving ease, temperament and length of herd life) are commonly included in breeding objectives among which milk production is the most important trait and also the main determinant of income to dairy farmers. On the other hand, age at first calving is economically important because it determines when an animal begins its productive life and therefore could influence the lifetime productivity of an animal. Moreover, age at first calving can be considered as a measure of heifer fertility performance associated with reproductive efficiency. The main aim of the present research is multivariate genetic analysis of first lactation milk yield at different calving ages of Iranian Holstein heifers in Khorasan province of Iran.