We investigated the growth rate enhancement in selective area metalorganic vapor phase epitaxy with a new surface boundary condition for the vapor phase concentration of source materials. In our model, the growth rate is proportional to the vapor phase concentration at a distance of the vapor phase mean-free path of source materials from the substrates, and the vapor phase concentration is laterally uniform at the top of the stagnant layer. This model predicts that the growth rate modulation produced by mask-patterning disappears at a specific pressure at which the vapor phase mean-free path reaches the stagnant layer thickness. We controlled the vapor phase mean-free path of trimethylindium in InP growth by varying the growth pressure from 6 to 180 Torr with a fixed total inlet gas flow rate to the reactor. Experiments showed a great reduction of the growth rate enhancement with the decrease in the growth pressure and the growth rate enhancement virtually disappeared at the lowest pressure. We also investigated the dependence of the stagnant layer thickness on the mask design. The experimental stagnant layer thickness related to the typical surface length, such as the period of mask-patterning.