Obesity, a rapidly growing threat to human health worldwide, is responsible for a large proportion of the total burden of disease. Therefore, obesity control could be a vital scheme to prevent many diseases. The aim of this study was to examine the activities and mechanism of Auricularia auricula-judae 70% ethanol extract (AAE) in preventing hypolipidemic and hepatic steatosis. A normal diet (ND) and a high-fat diet (HFD) with or without 0.1% (w/w), 0.3% (w/w), and 1% (w/w) AAE were given to male C57BL/6 mice. Plasma lipids and liver enzymes were measured and tissue sections of liver were examined. Further mechanistic studies of mouse 3T3-L1 adipocytes were performed in vitro by verifying triglyceride, glycerol, and glycerol-3-phosphate dehydrogenase activity and messenger RNA expression of adipogenic and lipogenic genes using reverse transcriptase polymerase chain reaction amplification. Body weight and adipose tissue mass were significantly reduced in mice fed an ND and a HFD plus AAE compared with mice fed an HFD. In AAE-supplemented groups, plasma lipids and liver enzymes decreased dose-dependently. AAE suppressed the expression of adipogenic/lipogenic genes (PPARγ, C/EBPα, FAS) in 3T3-L1 cells without cytotoxicity. These findings suggest that AAE may reduce the risk of hepatic steatosis by modulating plasma lipids via the regulation of adipogenic/lipogenic transcriptional factors. AAE may have interesting applications to improve plasma lipids and liver enzymes.