For pure molybdenum nitrided at 1373 K in NH, gas, microstructural observations of a molybdenum nitride layer were conducted through a transmission electron microscope. The molybdenum nitride layer consisted of two molybdenum nitrides: an outer one of gamma -Mo2N and an inner one of beta -Mo2N. A great number of (011)[0(1) over bar 1] type twins were observed in the beta -Mo2N phase, whereas the microstructure of the gamma -Mo2N phase was almost free from lattice defects except for slightly observed (111)(112) type twin.