This study was conducted to investigate the effects of Mesangi (Capsosiphon fulvescens, CF, a green alga) on lipid metabolism in rats, which was examined by analyzing the lipid composition in serum. Male Sprague-Dawley rats were assigned to one of three groups: the basal diet, high cholesterol, and high cholesterol supplemented with 5% dry Mesangi powder (CF-supplemented group). The body weight gains and food efficiency ratios of the rats fed the CF-supplemented diet were lower than those of the rats fed the basal diet. The levels of total lipid, total cholesterol, and LDL-cholesterol in serum were reduced in the CF-supplemented group as compared to the cholesterol group. However, the level of HDL-cholesterol in blood increased with the addition of CF to the diet. Furthermore, levels of total lipid and cholesterol of liver in experimental group fed CF were significantly lower than the cholesterol group. A decrease in leptin expression levels was observed in the CF-supplemented group as compared to the cholesterol group. These results suggest that the addition of CF in hypercholesterolemic rats has an effect on the improvement of serum and liver levels of cholesterol, which may be related to the regulation of the atherogenic index and lipid metabolism in rats fed CF.