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One of the most serious concerns facing developing countries is high concentrations of ambient particulate matter (PM2.5). Concurrently, climate change has also been a major challenge for countries around the world. Energy systems are dominant emission sources of both PM2.5 and carbon dioxide (CO2). This study investigates cost-efficient energy system transitions for individual or dual targets of climate change mitigation and PM2.5 concentration reduction by a global energy systems model. We set two levels of mitigation efforts for each of the CO2 and PM2.5 emissions, whose stringent one corresponds to the long-term target of the Paris agreement and current national air quality standards, respectively. For PM2.5 reduction, a combination of moderate improvement in energy efficiency and a transition from coal to gas and renewable energies and a significant deployment of end-of-pipe measures for scrubbing air pollutants is shown to be among the most cost-efficient strategy. For CO2 reduction, drastic improvement in energy efficiency and a rapid transition from coal to gas, renewable and nuclear energies is the most cost-efficient strategy, in contrast. There is a larger co-benefit on PM2.5 concentration reduction from CO2 reduction measures than the converse, and the co-benefit varies regionally. Developing countries such as India have a huge potential of co-benefits, and energy efficiency improvement and fuel switching are key measures to bring them. The simultaneous achievement costs of the dual targets are smaller than the sum of individual achievement costs, and the cost reduction varies significantly depending on the level of each mitigation target.