For environmental protection, it is necessary to further reduce harmful emission of the internal combustion engines. In this research, in order to reduce the PN (particle number), the flow inside of the injector was analyzed. As a result, we got guidelines to reduce tip wetting. The guidelines are as follows. 1) Reduction of the fuel flow velocity in the vertical direction to the hole axis around upstream of the injection hole. 2) Selection of hole arrangement and hole angle so as to suppress separation of flow inside the injection hole. 3) Improvement of radial air flow velocity on the tip surface to reduce tip wetting. We confirmed the reduction of PN with the optimized injector samples by the actual engine test.