Droplet collision in a diesel spray has been studied through the observations of a binary collision system with two equivalent droplets of the same liquid. Moreover, conventional CFD (Computational Fluid Dynamics) codes only consider the coalescence and the stretching separation of two identical liquid droplets. DWI (Direct Water Injection) system recently introduced to reduce NOx emission from marine diesels, however, could have considerable collisions between the droplets of the different liquid, that is, between water droplets and fuel droplets since it has near co-axial nozzle layout and water is usually injected under much lower pressure than fuel. In this study, the criteria of the collision outcomes between immiscible droplets were newly derived theoretically and implemented into KIVA3 code. These criteria divided bounce, coalescence and reflexive or stretching separation regimes. The effective entrainment of water into fuel spray was numerically captured in the DWI system of the two-needle injector by introducing concentric fuel (inner) and water (outer) sphere after coalescence or reflexive separation. The predictions were partially confirmed through observation of the actual DWI spray in a constant volume bomb.