Risk assessment of CO2 leakage from geologic sequestration sites is unavoidable for the implementation of the technology. However, the nature of the risk is different from well-known other toxic materials due to the complexity and uncertainty of the earth crust structures, long life time of CO2, and wide spatial distribution of leakage and its risk. In this paper, the characters of the risk and involved factors are summarized, and risk assessment schemes arediscussed. Impacts of the leakage, flow paths, mechanism, and categorization of the storage structures are essential to assess the risk. A quasi quantitative analysis method is also proposed.