A sulfated polysaccharide extracted from Gracilaria lemaneiformis (GLP) with a prominent effect in regulating lipid metabolism was isolated. The molecular weight was 31.5 kDa and it was composed mainly of galactose, glucose and xylose. Fourier-transform infrared (FT-IR) spectrum and nuclear magnetic resonance (NMR) analysis suggested that GLP was composed of the following repeating unit: [3-β-Gal-4(OSO3)-1→4-α-3,6-anhydrogal-2(OSO3)-1→]. GLP could significantly decrease serum total cholesterol, triglyceride and free fatty acid levels and lower alanine aminotransferase and aspartate aminotransferase activities in high-fat-diet mice. Additionally, GLP could keep the body weight and attenuate accumulation of fat surrounding the liver and epididymis induced by high-fat diet. Results of RT-PCR indicated that GLP might regulate lipid metabolism and accelerate free fatty acid oxidation by up-regulating the expression of the PPARα, ACS and CPT1a gene. The present study suggests that GLP may be potentially useful for regulating lipid metabolism.