The anti-ulcer properties of astaxanthin fractions such as total carotenoid and astaxanthin esters from Haematococcus pluvialis were evaluated in ethanol-induced gastric ulcers in rats. Since oxygen radical release is a pathogenic factor of ethanol-induced gastric damage, astaxanthin — a free radical scavenger, was investigated as a potential ulcer preventive agent. Astaxanthin fractions — total carotenoid and astaxanthin esters were orally administered to experimental rats at 100, 250 and 500 μg/kg b.w. prior to ulcer induction. Alcian blue binding assay indicates that, total carotenoid and astaxanthin esters at 500 μg/kg b.w could protect gastric mucin ~40% and 67% respectively. Pre-treatment with astaxanthin esters, also resulted in significant increase in antioxidant enzyme levels — catalase, superoxide dismutase, and glutathione peroxidase in stomach homogenate. Histopathological examination substantiated the protective effect of astaxanthin in pre-treated rats. The increased antioxidant potencies such as free radical scavenging activity with an IC₅₀ of ~8 μg/ml and reducing power abilities (59×10³ U/g) in vitro, reveal that H. pluvialis astaxanthin may protect gastric mucosal injury by antioxidative mechanism. In addition, ~23 fold increased lipoxygenase-inhibitory property, in comparison with standard astaxanthin and significant H⁺, K⁺-ATPase-inhibitory activity of astaxanthin esters, in comparison with known proton pump blocking anti-ulcer drug — omeprazole, may envisage the potential gastroprotective effect by regulating the gastric mucosal injury and gastric acid secretion by the gastric cell during ulcer disease.