The supplemental fats can reduce milk fat and protein percentage and increase milk yield, because the addition of fat can interfere with ruminal fermentation result in decreased fibre digestion. By the way, forage particle size, however, is closely related to ruminal lipid metabolism in several ways including increase in the size and consistency of ruminal mat, increase ruminal mean retention time of the particulate and lipid and its availability for microbial attack and to reduce the rate of lipid hydrolysis and biohydrogenation, which could potentially lower and release of antimicrobial fatty acids. Jenkins et al. (1998) reported a tendency for increase production of FCM when tallow was added to diets with shortcut hay, which suggests at least a limited role of hay particle length. In addition several researchers reported that reduction of forage particle size cause increased DMI, milk yield and milk protein but decreased chewing activity, rumination, Intake time, rumen pH, milk fat percentage and fibre digestibility (Teimouri Yansari et al., 2004). Therefore, this study was conducted to evaluate the effects of alfalfa hay particle size and soybean oil on nutrient digestibility, chewing activity, milk yield and composition of early lactating Iranian Holstein cows.