The cholesterol and fatty acid levels in serum and liver were compared in 4-week-old Sprague-Dawley male fed by addition of polymannuronate (M, 5.0％), polyguluronate (G, 5.0％), and polymannuronate and polyguluronate (MG, 2.5％ to each) with by addition of cholesterol (1.0％). Feed efficiency by the addition of M, MG, and G was lower than cholesterol fed group (p＜0.01). The liver weights were less in M, MG, and G fed groups than in cholesterol fed group (p＜0.01). Triglyceride levels in serum and liver were 58.2∼77.4％ and 51.5∼65.5％ lower in M, G and MG fed groups than cholesterol fed group, respectively. Total-, LDL-, and free-cholesterol levels in serum and liver in M, MG, and G fed group were significantly lower than cholesterol fed group. The cholesterol levels were the most reduced in M fed group. However, HDL- cholesterol level in serum was increased in M, MG, and G fed group (p＜0.01). The of polyene levels were 47％ higher in serum and 76％ in liver in M fed group than in cholesterol fed group. The activities of GOT and GPT were lower in M, MG, and G fed group than in cholesterol fed group (p＜0.01). Above the results demonstrate that supplementation of low molecular polymannuronate in diets improve physiologically lipid composition in serum and liver.