The N-doped diamond films have been investigated by transmission electron microscopy (TEM). TEM images clearly reveal that both the density and distribution of the planar defects are greatly affected by the N/C ratio (the ratio of N atoms to C atoms in the source gas). As the N/C ratio increases, much more planar defects are formed. Moreover, the phenomenon that a heavily nitrogen doping leads to the generation of the amorphous domains in diamond lattice is directly confirmed by TEM images for the first time, and the average size of these amorphous domains is on the order of nanometers. Based on the TEM observation results, the nitrogen doping effects on the crystallinity variation of chemical-vapor-deposited diamonds are discussed.