Objectives : Effects of Rhizoma Alismatis extract on lipid composition and TBARS (thiobarbituric acid reactive substances) formation were investigated in rat fed high fat diet. Methods : Seventy male Sprague-Dawley rats were used in these experiments and housed with food and water ad libitum and Twenty rats were selected healthy over 400g body weight and fed high fat diet for 8 weeks. Rhizoma Alismatis extract was administerd at dose 100mg/kg/day, 200mg/kg/day and 300mg/kg/day p.o. for 4 weeks. Then Triglyceride, Total cholesterol, HDL-cholesterol, LDL-cholesterol, TBARS concentration and ALT(Alanine Aminotransferase), AST(aspatate transaminase) activities were determined. Results : Plasma total LDL-cholesterol showed a tendancy to decrease in all treated group(P<0.05), however plasma HDL-cholesterol, total cholesterol, triglyceride showed no significantly different. Rhizoma Alismatis extract feedings resulted in a decrease in plasma TBARS(p<0.05) concentration and AST, ALT activites but contents of TBARS in liver showed no significantly different among all the treatments. Conclusions : Reviewing these experimental results, it appears that Rhizoma Alismatis extract was effective for inhibiting lipid peroxidation and decreasing LDL-cholesterol concentration.