Surface morphologies and fundamental characteristics of molybdenum nitride films deposited by reactive dc magnetron sputtering were studied for application to Cu diffusion barrier. A phase transformation from Mo to $gamma$-Mo$_2$N phase at 0.5$N_2$ flow ratio.($N_2$/(Ar+$N_2$)) equal to and larger than 0.2, whereas a second phase transformation to $gamma$-MoN phase at 0.5 N2 flow ratio, With the variation of the N2 ratio the surface morphologies of the films were generally smooth except the cases of 0.2 and 0.3$N_2$ gas rations, where build-up of film stresses occurred. $gamma$-Mo$_2$N film was found to crystallize at the deposition temperature of 40$0^{circ}C$. The surfaces of $gamma$-Mo$_2$N films deposited up to 40$0^{circ}C$ were smooth, but the film deposited at 50$0^{circ}C$ had very rough surface morphology. It seems that this was due to the building-up of thermal stresses at the high deposition temperature, which might lead to hillock formation.