The objective of operational reliability evaluation is to assist operators to make decisions on power system operation. Therefore， it is important to set a criterion for decision-making. However， it is difficult to make decision based on the common reliability indices directly. The concept and indices of power system operational reliability cost/worth are proposed， and a new criterion of minimal expected social cost can be used to help operators choose best operating mode. Models and algorithms of operational reliability cost/worth evaluation of composite generation and transmission system are established. By these methods， power system operational reliability cost and benefit can be analyzed. The proposed models and algorithms have been used to choose optimal schemes of reserve allocation on IEEE RTS-79 reliability test system， and their effectiveness is verified.