As power industry evolves into Smart Grid scheme, previously closed power systems are being integrated into public communication networks. It increases the controllability and efficiency of the system, but also accompanies many cyber threats having existed in the Internet to the SCADA system. Therefore it is required to apply security countermeasures to the Smart Grid, which brings about investment costs. There have been few approaches to assess risks from cyber attack especially in electric power industry. So this paper proposes a methodology to assess quantitative impacts of various types of cyber attacks to a power system, and also shows the feasibility of the method through a case study.