Microcrystalline diamond (MCD) and fine-grained diamond (FGD) films are deposited on silicon nitride (Si3N4) inserts using the hot filament chemical vapor deposition (HFCVD) method. Scanning electron microscope (SEM), X –ray diffraction (XRD) and Raman spectrum are employed to characterize these as-deposited diamond films. Cutting performance of as-fabricated CVD diamond coated Si3N4 inserts is examined in dry turning glass fiber reinforced plastics (GFRP) material, comparing with the uncoated Si3N4 inserts. The results indicate that the tool failure is mainly attributed to its severe flank wear, which is caused by continuous friction and impact brought by many hard SiO2 particles distributed in the GFPR work piece. The lifetime of Si3N4 inserts can be prolonged by depositing MCD or FGD films on them and the FGD coated insert shows better wear resistance than the MCD coated one.