Whey, a by-product of the dairy industry, is primarily used for protein and energy sources in livestock diets due to its high nutritional value, which equivalent to grain corn and barley. In the current study, whey powder was added to lamb drink water to evaluate its effects on performance and in vivo carcass measurements. Twenty-four non-castrated healthy male Merino lambs with an average of 3 months weaning age and 23.44 ± 0.62 kg (mean ± SE) initial live weight (LW) used in this study.