Experimental results on growth morphology in N‐polar GaN grown by low‐pressure metalorganic chemical vapor deposition (LP‐MOCVD) were analyzed using a thermodynamic supersaturation model for gallium. Smooth N‐polar GaN films with 1 nm RMS roughness were always obtained under extremely low Ga supersaturation in the vapor, although the growth conditions were seemingly different. It was found that increasing H2 partial pressure during the GaN growth played a role in significantly lowering the Ga supersaturation, since H2 is a product in the formation of GaN. The degree of Ga supersaturation was also controlled by adjusting the partial pressure of Ga species in the vapor. The surface‐diffusion theory dealing with step dynamics described by Burton, Cabrera, and Frank (BCF) was also used to relate Ga supersaturation to the observed smooth N‐polar GaN growth. These findings showed that a supersaturation model encompassing all major MOCVD growth parameters can be used to predict the smooth N‐polar GaN growth conditions. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)