As its hardness and wear resistance being approximate to that of natural diamond, polycrystalline diamond (PCD) is a kind of perfect tool material for cutting Si-Al alloy pistons. In the turning process of Si-Al alloy pistons, the normal wear of PCD tools is a continuous and gradual process, which mainly is hard grain wear, as well as adhesion wear, diffusion wear, and chemical wear. The breakages of PCD tools include collapse, crash and flake etc, which occur mainly in discontinuous turning process. Experimental results and production statistic indicate that about 40~55% of PCD tools failure is breakage in discontinuous turning process of Si-Al alloy pistons. Reducing impact load is the main method to avoid tool breakage.