Seasonal variations in body weight and composition of body fat and water, as well as energy contents were measured in field Brandt's Voles (Lasiopodomys brandtii) and Mongolian Gerbils (Meriones unguiculatus). Both species had the highest body weights in spring, highest fat and energy contents in autumn, and highest water contents in summer. However, patterns of seasonal variations in body weights are different between Voles and Gerbils. Body weight variations were affected by both water and fat contents, which might be influenced by ambient temperature and food quality.