In this study, we prepared a crude polysaccharide from a Celluclast enzymatic extract of Ishige okamurae (IOP) and evaluated its radio-protective effects in immune cells of gamma ray-irradiated mice. First, the extraction yield of IOP was approximately 14.5% and contained major contents of galactose (20.8%), glucose (18.1%), and mannose (14.8%). Also, IOP dose-dependently increased the proliferation of splenocytes without cytotoxicity. In addition, IOP significantly enhanced the proliferation of 2 Gyirradiated splenocytes whereas reduced the intracellular ROS production. In further study, IOP improved the ratio of spleen weight/body weight by increasing the number and proliferation of splenocytes, compared to only 2 Gy-irradiated mice. Especially, IOP increased the population of CD8 + cytolytic T cells, whereas reduced the population of Gr-1 granulocytes. Interestingly, IOP recovered the ratio of CD4+ helper T cells/CD8 + cytolytic T cells, compared to the only 2 Gy-irradiated mice. According to these results, we suggest that IOP protected mice as improving the peripheral immune system damaged by gamma ray irradiation in mice.