Abstract: A polycrystalline diamond tool was selected to investigate the influence of fraction and size of reinforcements on the machined surface of SiC-particle-reinforced aluminum matrix composites.There are three removing mechanisms,i.e.pulling out,crushing and cutting off during turning this kind of material.When the composite have a higher volume/weight fraction and bigger mean size of SiC particulate reinforcement,the number of particles that were pulled out or crushed is bigger too.This resultes in a high value of the machined surface roughness.A better ultra-precision turning surface that have fewer pits and cracks could be expected when SiCp was removed mainly in the mode of cutting off mechanism.