With the aim of improving environmental management in the Asia region, several Japanese municipal governments have engaged in international co-operation with local governments in developing countries in the region. This kind of co-operation is not usually considered within the mandate of local governments, and it has occurred despite increasing fiscal constraints. This study examines the motivations behind international environmental co-operation of 12 Japanese cities, and it finds that they can be divided into two types: materialistic and idealistic motivation. Materialistic motivation emphasises the utilisation of local human capital for environmental management, promotion of environmental businesses, or prevention of trans-boundary pollution, while idealistic motivation includes a sense of pride in sharing the responsibility of environmental protection and conservation, or a sense of sharing global issues beyond the national border. The cities of Kitakyushu and Yokohama are representatives of the two patterns. Socioeconomic factors such as exports, foreign residents, foreign visitors, and fiscal capacity, although expected to enable co-operation, were found to be not highly correlated with it.