Lentinan (LNT), a type of polysaccharide derived from Lentinus edodes, has manifested protective effects during liver injury and hepatocellular carcinoma, but little is known about its effects on nonalcoholic fatty liver disease (NAFLD). This study aimed to investigate whether LNT can affect the progression of NAFLD and the associated mechanisms. C57BL/6J mice were fed a normal chow diet or a high-fat diet (HFD) with or without LNT (6 mg/kg/d). AML12 cells were exposed to 200 mu M palmitate acid (PA) with or without LNT (5 mu g/mL). After 21 wk of the high-fat diet, LNT significantly decreased plasma triglyceride levels and liver lipid accumulation, reduced excessive reactive oxygen species production, and subsequently attenuated hepatic apoptosis in NAFLD mice. These effects were associated with increased PPAR alpha levels, a decreased Bax/Bcl-2 ratio, and enhancement of the antioxidant defense system in vivo. Similar effects were also observed in cultured cells. More importantly, these protective effects of LNT on palmitate acid-treated AML12 cells were almost abolished by PPAR alpha knockdown. In conclusion, this study demonstrates that LNT may ameliorate hepatic steatosis and decrease oxidative stress and apoptosis by activating the PPAR alpha pathway and is a potential drug target for NAFLD.