This study was carried out to investigate the effect of Lythrum salicaria L. ethanol extract on anti-obesity effects in rat fed a high fat diet for 8 weeks to induce obese rat model. Male SD rats were divided into normal group, control (high fat diet) group, positive control (Garcinia Cambogia extracts) group, high fat group supplemented with ethanol extracts of Lythrum salicaria L. (EELS). The body weight gain and control (high fat diet) were increased by a high fat diet, but decreased in the EELS. At the end of the experiment, the body weight in high fat diet groups was higher than that of normal diet group, while the body weights of EELS and positive control group were significantly reduced by 16.62%, as compared with that of high fat diet group (p < 0.05). The levels of serum triglyceride, total cholesterol in EELS group were significantly decreased as compared with high fat diet group (p < 0.05). The liver and mesenteric adipose tissue weights of control (high fat diet) increase than that for normal group, whereas EELS and positive control group were significantly decreased (p < 0.05). Levels of triglyceride in liver were significantly lower in EELS group than those in high fat diet group (p < 0.05). These results indicate that Lythrum salicaria L. extract may improve lipid metabolism and reduce fat accumulation and body weight.