This paper proposes a method of cost-effective preventive maintenance planning for all regions of the Provincial Electricity Authority of Thailand (PEA). The simplified reliability centered maintenance (S-RCM) technique is applied to select the suitable supplementary preventive maintenance activities for each region. First, the reliability improvement opportunity (RIO) dataset of power distribution systems are assessed for each PEA region. The regional RIO datasets are used for preventive maintenance planning in order to minimize the customer-minute of interruptions (CMI) and to minimize the supplementary preventive maintenance cost of the whole PEA system. The cooperative game theory is chosen to facilitate the above procedure to obtain the most cost-effective preventive maintenance program.