In this paper a mathematical model is developed for prediction of important performance parameters and emission of major pollutants from a Direct-Injection (D.I) diesel engine. The model has ability for simulating performance and emission of four-stroke D.I diesel engines by taking into account the closed cycle (compression stroke, combustion process and expansion stroke). By developing a fuel injection system (FIS) submodel, variation of injection pressure during the injection period is considered. Also in this code, time period of ignition delay is predicted by considering the physical and chemical processes. The combustion submodel is a quasi-dimensional phenomological model and is based on well know Integral Multi-Zone Combustion Modelling (IMZCM) concept. It takes into account, on a zonal basis, details of fuel spary formation, air-fuel mixing, and air swirl. The emission submodel uses the chemical equilibrium as well as the governing kinetics rules for calculation of pollutants formation such as Nitrogen Oxides (NOx) and soot particles. For calculation of NOx formation rate, extended Zeldovich mechanism has been used. For evaluating soot particles concentration, the amount of net soot is calculated by considering the difference between the rates of soot formation and its oxidation inside the zones. The results obtained from the model are compared with the experimental conditions of the typical medium and high speed D.I. diesel engines. The comparison shows that, there exist good agreement between the predicted and the experimental results.