Milling tests for SiCp/2009Al composites were performed by using Polycrystal Diamond(PCD) tools at the cutting speed of 600-1 200 m/min and the mechanisms of tool wear were investigated. A Scanning Electron Microscope (SEM) was used to examine the machined surfaces and tool wear land, an X-ray diffractometer (XRD) was used to analyze the substances on the machined surface and the laser Raman spectra and Energy Density Spectrometry(EDS) were taken to analyze the elements on the tool wear land. The results show that the tool grain breaking-off, abrasive wear, chipping and the peeling caused by the high frequency impact and the scrapt of SiC particles are the prevalent wear patterns. When high volume fraction materials or heat treated materials are milled, or the milling is at a higher cutting speed, micro-cracks will form on the PCD tools. Furthermore, volume fractions, average sizes of SiC particles, tool grain sizes, cooling and heat treating conditions have significant influence on the tool wear. It concludes that the life of PCD tools is in the range of 70-240 min. The micro-cracks are produced on the tool flank under the combined effects of SiC particle impact, cutting vibration and heat impact. Moreover, the aluminum and copper can be diffused into the tool matrix under the very high cutting temperature and cutting forces and the trivial graphitization is taken place on the PCD tools due to the catalysis of copper in the aluminum matrix.