In recent years increasing milk production in Iran caused to increase nutrient requirement of dairy cattle. In other wise in semiarid countries such as Iran providing forage requirement of cattle is limited by deficiency in forage resources. So it is better to use plants that they have low water requirements, high growth rate and high forage yield per hectare. Rapeseed is reasonably widely adapted and performs well in many areas in Iran. In spite of its low water requirements and high growth rate, its cultivation season is matched with environmental condition in Iran. Rapeseed (Brassica napus L.) recently has received consideration as a forage source for livestock. Its use has been restricted by numerous antinutritions such as glucosinolates and Erucic acid (Wilkes et al., 1987). Fales et al. (1987) reported a 100-fold reduction in Glucosinolate concentration when rapeseed forage underwent ensiling. The main objective of this study was to evaluate effects of rapeseed silage on Holstein dairy cow’s performance.