$Dioscorea$ $batatas$ Decne is a type of yam that eaten raw and used as a traditional oriental medicine in Asian countries. In this study, we evaluated the functional effects of yam water extracts on body weight levels and serum lipid concentrations in mice fed high fat diet. Mice were divided into four groups: normal diet control (ND), high fat diet control (HFD), HFD+yam extract 100 mg/kg (HFD-Y100), and HFD+yam extract 200 mg/kg (HFD-Y200). Yam extract was administrated orally to mice fed a high fat diet for 5 weeks. Treatment with yam extract significantly reduced body weight levels and energy efficiency in a dose-dependent manner in HFD-fed mice. Yam extract also attenuated serum levels of triglyceride, total cholesterol, and LDL-cholesterol, as well as organ weights of liver and abdominal adipose tissue in mice fed a high fat diet. Moreover, blood levels of glucose, insulin, and leptin significantly decreased in a dose-dependent manner upon administration of yam extract. Blood activities of GPT, GOT, and LDH were lower in the yam extract-treated groups compared to the HFD group. These results indicate that yam water extract may reduce elevated body weight and serum lipid concentrations in mice fed a high fat diet, suggesting its usefulness as a functional food for reducing body fat and hyperlipidemia.