Abstract: Coated carbide tools are now playing a more and more important role in production engineering. It appears to be a problem, to both the makers and the users of coated carbide tools, how to optimize the selection of the substrate materials in accordance with the selection of coating layers. Taking as examples the cemented carbides varieties WC-Co-TiC and WC-Co-TiC-TaC (NbC) kinds, which are the very common materials as substrates of the coated carbide tools of steel cutting grades, this paper investigates the characteristics of their properties and their effect on the failure mechanism of coated carbide tools. Based on some test results and theoretical analyses,some priciples for the selection of substrate materials in specific cutting conditions are proposed. The necessity and possibility of the design of special substrate materials are also discussed.