In order to make sure the details of influences of grain size on the performance of diamond thin film, different trademarks of tungsten carbide tools with different scales of substrate grain size were selected. Scanning electron microscopy (SEM), Raman spectroscopy, indention tests and surface scanning profilometer showed that WC grain size played an important role in determining the performance of diamond films deposited on the tungsten carbide substrates. Therefore, an appropriate combination of pretreatments, substrate grain size and deposition parameters should be selected. The tribological pin-on-disc tests have been conducted. The quantitative relations between the friction coefficient and the surface roughness were investigated. The experimental results have shown that the surface roughness of diamond coatings has appreciable influences on the friction coefficient, especially on the initial friction. There exists an optimal value of surface roughness of diamond-coated tools with regard to cutting tool life.