Nitrogen-doped and undoped diamond films grown by microwave plasma chemical vapor deposition (CVD) were etched by electron cyclotron resonance (ECR) plasma with asymmetric magnetic mirror field. The influences of nitrogen doping on the etching characteristic of CVD diamond films are studied by scanning electron microscope (SEM), X-ray photoelectron spectroscopy(XPS), and surface roughness measuring instrument; and the etching mechanism is explicated in detail by etching models. It is found that the crystal edges are dramatically etched for the nitrogen-doped diamond film, while the (111) facets are etched and crystalline grains collapse for the undoped diamond film. And after etching by ECR plasma for 4 h, the nitrogen-doped diamond film surface roughness decreases from 4.761 μm to 3.701 μm, while the surface roughness of the undoped film decreases from 3.061 μm to 1.083 μm. The results indicate that nitrogen doping has great influence on the etching characteristic of the CVD diamond films. Nitrogen-doping deteriorates the film quality and increases the defect density in the crystallites. And the defects distributed in the crystal edge lead to dramatically etching of the crystal edge. Compared with the nitrogen-doped diamond film, the defect density in undoped diamond film is relatively low and the distribution of defects is comparatively uniform, resulting in the fact that (111) facets would suffer from oxygen cyclotron ion beams bombardment and so grains of the film collapse. The reason why the surface roughness of nitrogen-doped diamond film decreases less than the undoped diamond film is that the movement of ions is affected by the electrons emitting from crystal edge, which weakens the ion bombardment on (111) facets.