The Fukushima nuclear power plant accident occurred in Japan on March 2011, initiated by tsunami following a great earthquake cause the discharged of radioactive materials into the environment. This accident received considerable attention for their effect on marine ecosystems. Sea urchins are model organisms in developmental biology research and their embryos are sensitive to toxins and used to study the developmental and cytological effects of anthropogenic pollutants and environmental stressors. In the present study sea urchin embryos were used as a model system to assess the effect of ionizing radiation on the viability and early development of marine invertebrate animals. Sea urchin embryos were culture in filtered sea water at 16 °C at the different developmental stages were irradiated with X-rays (70 kV, 0.2 mm Al filter, dose rate=1.46 Gy/min) and further incubated in filtered sea water at 16 °C. Irradiation of embryos at the different stages of development (32cell, mid-gastrula and early Pluteus larva) at doses up to 30 Gy did not reduce the viability of embryos. However, irradiated embryos exhibited dose-dependent developmental abnormalities. Typical abnormalities observed for gastrula embryos were delayed development and a reduced number of primary and secondary mesenchyme cells and those for mid Pluteus larva were delayed development, skeletal abnormalities, separated body rod tips and fused arms.