Spray combustion is utilized in a number of engineering applications such as energy conversion, military industrial, furance and propulsion devices. Current work focused on the effect of liquid fuel droplet diameter on the efficiency of the combustion chamber and formed emission such as NOx and CO in a two-dimensional axisymmetric combustion chamber. The discrete phase model approach employed for simulating Combustion. The gas phase is simulated using an Eulerian approach; while the droplets are treated with a Lagrangian method. The coupling between the two phases and effect of radiation is considered. The mixture-fraction/probability density function (PDF) equilibrium chemistry model is used to predict the combustion of the vaporized fuel. Also, the conservative equations of mass, momentum and energy in the turbulent flow field were solved in conjunction with the k–ε two equation turbulence model. A numerical simulation was carried out to study the influence of droplet size on the formation and emission of NOx and other contaminants. This effect was investigated under different droplet diameter and type of injection. The following conclusions be drawn: Smaller droplets produce higher NOx emission than the larger ones. Larger droplets produce higher CO than Smaller ones.