Theoretical expressions of the epitaxial lateral overgrowth rate in the metalorganic chemical vapor deposition (MOCVD) have been formulated in this paper, with respect to two separate processes: vapor phase diffusion and mask surface diffusion. In the case of InP deposition on GaAs substrate, a parametric study was accomplished in order to determine the impact of the mask/window width to the growth rate. The model, which uses a new parameter “effective mask length” Lmask, reveals that the key factors determining the growth rate are mask/window width and mask width/effective mask length. This model can be used as a tool for predict the growth conditions leading to expected growth rate.