This study was designed to observe the effect of Undaria Pinnatifida (U. P.) Powder on serum and liver lipids concentration in rats. Adult male rats of Sprague-Dawley strain were divided into four diet groups: one control diet and the other three diets in which U.P. powder was mixed with the control diet at the rate of 1%, 2% and 3% respectively. The results of the above observation were as follows : Serum and liver total lipids in 3% U.P. group were lower an those in control group at the 6th week. There was no difference in serum and liver total cholesterol among the diet groups. Serum HDL-cholesterol and HDL-cholesterol per total cholesterol ratio in 3% U.P. group were higher than those in control group at the 3rd and 6th week. Serum triglyceride in 3% U.P. group was significantly lower than that in control group at the 3rd week. Serum phospholipid in 1% U.P. and 3% U.P. group was significantly lower than that in control group at the 3rd week, but there was no difference among them at the 6th week.