Nowadays new water supply projects have been on the decline as the water-power constructions have saturated, which means that the existing power equipment have slowly aged and they require more efforts to maintain the system performance. An effective asset management method of power equipment has become a great necessity from both economical and technical aspects. To be balanced, the asset management should look into all three parts: management, engineering, and information. The purpose of this paper is to study a Risk-Based Maintenance (RBM) matrix method through the deterioration evaluation algorithm for an efficient reliability assessment of oil-immersed power transformers by considering both asset management and technical evaluation. Make use of this result, the equipment will be decided to be replace or repair otherwise on service.