Polycrystalline diamond (PCD) tools were used in high speed machining of SiCp/2009Al and TiCp/TC4 composites. The results showed that milling forces decreased with the increasing cutting speed, and that the force value of vol.20%SiCp/2009Al was less than that of the matrix. The machined surface roughness Ra is less than 0.8μm for all the milling experiments until cutting time was 180 minutes. The machined surface included some defects such as scratches, craters, micro cracks and so on. Surface finish can be improved through increasing cutting speed. When milling, gradual flank wear and slight edge chipping took place on the PCD tool flank. But no obvious abrasive wear was observed. However, the PCD tool suffered from significant abrasive wear in the flank when turning. A flow type chip was formed either for milling or turning when vol.20%SiCp/2009Al was used. Some voids and micro cracks were found in the chip. For TiCp/TC4 composite, the chip took the form of interrupted saw-toothed due to the dynamic behavior of micro crack. Although PCD tool was very suitable for high speed machining of SiCp/2009Al, its geometry parameters must be rightly chosen for TiCp/TC composite, or else sever edge chipping would take place in a few minutes.