By choosing appropriate annealing temperature and annealing time, diffusion couples made of aluminum bronze and tool materials were prepared to simulate the diffusion and phase formation during the actual machining. These results were compared to those obtained from turning experiments. In agreement with results obtained from diffusion couple experiments, all turning experiments showed that major elements in both tool materials and workpiece diffused into their counterparts. The diffusion of C away from the tool will reduce the tool strength. It is helpful to use diffusion couple results to comprehend the diffusion effect in the actual turning process.