Tool wear is a problem in machining nickel-based superalloy Inconel 718, and it is thus of great importance to understand and quantitatively predict tool wear. The experiments of machining Inconel 718 with cemented carbide tools (uncoated and coated) were carried out. Some new observations and analysis of tool wear through CCD, SEM and EDS were done. The results showed that at low cutting speed, the built up edge (BUE) formed easily and dropped at last which caused severe cutting tool breakage. When cutting speed came to 30m/min, the main reason that caused cutting tool wear was that the tool material fell off from the tool body at the form of wear debris. What’s more, the element diffusion between tool and workpiece and oxidation reaction all accelerate the formation and the cast of the wear debris.