Nodular defects appear frequently in deposited films and coatings, especially in those deposited under higher deposition rate and lower substrate temperature. In order to understand the mechanism and to predict the effects of process parameters, a modified two-dimensional ballistic model of deposition growth has been developed and programmed for computer simulation. In this model, both scattering of depositing particles and surface diffusion of deposited particles are allowed. Simulated experiments have been carried out with respect to the effects of deposition rate, surface diffusivity of deposited particles, degree of scattering of depositing particles and presence of substrate asperities. Morphological features of the nodular defects are reproduced by the simulation. The results show that higher deposition rate, lower surface diffusivity of deposit and higher degree of scattering of depositing particles favour formation of nodular defects. Asperities on substrate are found usually responsible for formation of nodular defects; however, under favourable conditions, nodular defects can form without presence of any substrate asperity.