A simplified mathematical model for predicting spray-swirl interaction and the evaporation process of the fuel spray in D.I. diesel engine is developed. In order to achieve computational efficiency, the present model conservation equations for mass and momentum of the fuel spray have been taken in integral form using similarity profiles for instantaneous concentration, velocity and temperature within the spray. It is assumed that the radial distribution of the fuel droplet in the normalized size range follows a well established concentration profile. The effects of the initial mean droplet diameter (SMDo)and the droplet size distribution parameter (n) on the subsequent evaporation history and their changes with evaporation time have been taken into account. The model can be used to calculate fuel spray characteristics such as spray penetration, spray tip velocity, spray trajectory, fuel evaporation history and preparation rate of the flammable fuel-air mixture. In addition, the model can effectively be employed to predict the effects of varying some of the engine operating variables on each of the above characteristics of diesel spray. It is found that as evaporation proceeds the mean droplet diameter and distribution parameter both increase with time. The predicted results of this model have been compared with the existing detailed analytical and experimental data and found to be in good agreement.