The nitridation behavior of molybdenum (Mo) films has been studied in relation to the crystal structure of the films. Vacuum evaporated Mo films are nitrided to form gamma-Mo2N by annealing in H-2/N-2 mixtures in the temperature region of 400-900 degrees C. The temperature region becomes narrow with increasing H-2/N-2 flow ratio, and the Mo films are not nitrided by annealing in only nitrogen. These results are interpreted as caused by the reaction process involving hydrogen reduction of molybdenum oxide followed by direct nitridation of Mo. During annealing, the Mo films show uniform nitrogen distribution throughout the film and the nitrogen content increases with increasing annealing time. This behavior is well explained by the grain-boundary nitridation model in which each columnar grain of a Mo film is nitrided from the periphery to the center of the grain.