Laminariae Thallus has been known to be effective for the treatment of obesity. Te show the effectiveness of Laminariae Thallus in a more scientific way, Laminariae Thallus extract was prepared and evaluated in high fat diet rats by measuring the changes of body weight and lipid metabolism as described briefly below. 200 g of crushed Laminariae Thallus was extracted with methyl alcohol. The extract was evaporated under reduced pressure to give 14.4g. For 10 weeks, control group rat were ingested high fat diet, while the test group rat were fed with a fat diet pius Laminariae Thallus extract. Normal group was fed with normal diet. 150 mg of Laminariae Thallus extract per 1 kg of body weight was added to the diet in a test group rats. Control group rats on a high fat diet gained weight significantly, whereas the test group rats on a high fat diet plus Laminariae Thallus extract gained weight less. Significant increase of liver weight caused in a high fat diet was also inhibited by the Laminariae Thallus extract treatment. Total lipid, triglyceride, phospholipid and total cholesterol levels of serum in the high fat diet rats were remarkably increased, whereas their levels on a high fat diet plus Laminariae Thallus extract were less increased. While serum HDL-cholesterol levels was remarkably decreased in a high fat diet, its level was less decreased in a high fat diet plus Laminariae Thallus extract. Furthermore, we observed that the activities of hepatic acetyl-CoA carboxylase and fatty acid synthetase increased in a high fat diet, while their activities in a high fat diet plus Laminariae Thallus extract were getting back nearly to the normal levels of normal diet rats. These result showed that the obesity caused by a high fat diet was effectively inhibited by a Laminariae Thallus extract. Our results also showed that the abnormal lipid metabolism caused by a high fat diet was effectively cured by adding Laminariae Thallus extract.