It is important to understand the influence of gas transport properties on the process of urea gas decomposition in the modeling of a urea-SCR system. The purpose of this study is to find the gas transport property that exerts the strongest influence on the process of urea gas decomposition in a urea-SCR system using a three-dimensional model with the laminar flow of incompressible viscous gas at a steady-state condition by the SIMPLE algorithm. It is found that the gas transport properties which have the strongest influence on the process of urea gas decomposition in a urea-SCR system are the density and molecular diffusivity. It is also found that the influence of density is stronger than the molecular diffusivity at low temperature ranges. In contrary, the molecular diffusivity has stronger influence at high temperature ranges comparing with the density. Consequently, the density and molecular diffusivity with temperature-dependent function should not be neglected in the urea gas decomposition modeling of a urea-SCR system.