In order to improve the working life of micro end-mill, the diamond coatings were prepared by hot filament chemical vapor deposition (HFCVD) method with various methane concentrations. Scanning electron microscopy and Raman scattering analyses were carried out to characterize the surface morphology and coating property. With the increasing of methane concentration, the secondary nucleation rate was enhanced and the diamond grain size was decreased, which helped the diamond grain to embed into cemented carbide substrate, and improved film-substrate adhesion. Excess methane concentration weakened the etching ability of atomhydrogen on graphite phase, leading to higher fraction of non-diamond phase among diamond boundaries. While the methane concentration was 3%, the diamond coating deposited on micro end-mill possessed high crystallinity, good adhesion, more compact grains and low non-diamond phase. There was also no delamination occurred at the edge of end-mill. This work not only benefit us from understanding the effect of methane concentration on deposition of diamond film on cemented carbide micro end-mill and the mechanism of diamond film failure, but also provides basis for optimizing the parameters for diamond deposition on end-mills.