The steps and strategies of power system restoration are discussed. The blackstart power source planning model based on minimization of social interruption cost is proposed to optimize the capacity and location of the blackstart unit. Two scenarios are investigated. One is that each blackstart power plant gives pairing assistance to one non-blackstart power plant. The other is interconnected power system. The time varying characteristics of customer interruption cost rates are also considered. A genetic algorithm is employed to solve the problem. Examples are shown to demonstrate the feasibility of the proposed method. The factors that influence the blackstart power source planning are concluded at the end.