The growing concern about diesel vehicle tail-pipe emissions (NOx, PM and other unregulated emission) has led to demands for stricter emission regulations in the future. The objective of Step I study of diesel project, that is a part of JCAP, is to investigate the effectiveness and future direction of diesel emission control technologies and diesel fuel properties, and two different types of research have been carried out: “existing-step research” using market vehicles and fuels, and “model-step research” using specially prepared vehicles and fuels. In the existing-step research, combinations of 5 diesel fuels and 16 vehicles/engines from small passenger cars to heavy-duty truck engines were tested. In the model-step research, combinations of 8 diesel fuels and 5 recent vehicles/engines and 4 after-treatment technologies were tested.Overall results showed that applying both of DPF and highly oxidative catalyst with lowering sulfur in diesel fuel was effective on reducing PM, THC, CO and unregulated emissions. Additionally, the influences of fuel properties on emissions from the variety of vehicles/engines were much clarified.