Water-soluble polysaccharide was extracted from Laminaria japonica, and its antitumor effect on mice bearing H22 liver cancer was investigated. The mice were inoculated with H22 hepatoma cells and randomly divided into four groups: three treatment groups that received 50, 100 and 150 mg/kg L. japonica polysaccharide (LJP) intraperitoneal injection and one control group that received equal volume of physiological saline. Intraperitoneal injection of LJP increased serum interleukin-2 and tumour necrosis factor-α levels, as well as tumour inhibition rate of mice, but decreased serum vascular endothelial growth factor level. Therefore, LJP exerts antitumor effect and can be used as a therapeutic agent for cancer.