To evaluate the cutting performance of CVD diamond coated silicon nitride insert, two kinds of CVD diamond films, namely microcrystalline diamond (MCD) and composite diamond films, are deposited on silicon nitride (Si3N4) inserts using the hot filament chemical vapor deposition (HFCVD) method. The characterization of as-deposited diamond films is conducted using the scanning electron microscope (SEM), optical interferometric profiler, X –ray diffraction (XRD) and Raman spectrum. The turning tests for as-fabricated MCD and composite diamond coated insert, as well as uncoated silicon nitride insert, are performed in dry turning aluminum silicon alloy, where the cutting speed, feed rate and depth of cut are fixed as ν = 2000 r/min, f=0.1 mm/r and ap=1 mm. The results demonstrate that the main tool failure is the tool wear. As compared with the uncoated silicon nitride insert, the woking life time of the diamond coated insert can be increased by a factor of above 7.