Metabolism trials were conducted on six wethers and four Brahman steers given Ruzi grass hay with different levels of soybean meal in order to compare the effects of protein levels on energy and nitrogen balances, and fiber digestion between the two animal species. Crude protein (CP) contents in four dietary treatments were 3.4%, 6.9%, 10.4%, and 13.9% with the different levels of soybean meal supplement. Digestibilities of crude fiber, neutral detergent fiber and acid detergent fiber were greatly improved in sheep by the supplement of soybean meal until the CP content in the whole ration reached 10%. The difference was more than 10 units in each fiber fraction. While in cattle, fiber fraction digestibilities in the animals given Ruzi grass hay without soybean meal supplement were relatively high. The values were then improved by the smallest amount of supplement (6.9% CP). Beyond this level, there was no effect of the supplement. Although digestibility of CP was lower in sheep when no protein supplement was given, it was higher in sheep when high protein diets were given (10.4 and 13.9% CP). The total digestible nutrient and digestible energy contents well represented overall features of the differences in nutrient digestibilities between sheep and cattle. With lower levels of soybean meal supplement, these values were lower in sheep than in cattle. While, with higher levels of the supplement, there was no difference in the values between the animals. Sheep are often studied as a model animal for cattle in order to examine nutritive value of feed resources. It was suggested in the design of feeding trials using sheep, that CP content of whole ration, in which a target feed resource is included, should be more than 10%.