To observe the effects of marine collagen peptides (MCPs) prepared from chum salmon (Oncorhynchus keta) skin on life span and spontaneous tumor incidence, Sprague-Dawley rats were fed diets supplemented with MCP at concentrations of 0%, 2.25%, 4.5%, and 9% (wt/wt) from the age of 4 weeks until natural death. There were 40 rats in each group (male:female ratio=1:1). The results showed that the MCP did not significantly influence body weight or food consumption of rats of either sex throughout the life span; it did dose-dependently inhibit the age-related decrease in the activities of antioxidant enzymes and the age-related increase in the levels of lipid peroxidation product in both sexes. MCP notably increased the mean life span, the life span of the last 30% of the survivors, and the maximal life span; it decreased overall spontaneous tumor incidence of both sexes with significance in the 4.5% and 9% MCP-treated male groups and 9% MCP-treated female group. Compared to the control group, the incidence of death from tumors was decreased in MCP groups in comparison with the control group of both sexes. Therefore, we concluded that MCPs dose-dependently increase life span and decrease spontaneous tumor incidence in Sprague-Dawley rats. Moreover, the antioxidative property of MCPs may be responsible for the increased life span and protection against tumor development.