Coated WC-Co based cutting tool materials are widely used in machining applications. However some of the coating techniques, such as physical vapor deposition (PVD), involve only the physical deposition of coating layers which are prone to bulk delamination in some applications. Therefore functionally graded microstructure through diffusion is an efficient and widely used methods to improve the performance of coated WC-Co tools. In this work ISO P25, WC-Co based inserts were subjected to a thermochemical diffusion treatment by boronizing in a molten salt bath. The process was performed at 900, 1000, and 1100 °C for 1, 2 and 4 h. SEM, X-ray, EDS and microhardness analysis were performed on the samples. It was found that boron containing hard phases of W 2 B 5 , CoB, W 2 CoB 2 and WCoB were formed within the boronized region, occupying the grain boundary areas. An average microhardness value of 3600 HK 0.01 was measured in the near surface region which is substantially higher than the value of 1418 HK 0.01 representing the inner sections of the material.