Cutting tools made of the WC-Co sintered carbides are now very popular and are widely used in machining of materials. However, there are numerous problems in this area which require more research and need to be studied further. This paper presents the results of an experimental study aimed at discovering the impact of the microstructure, particularly of the tool substrate grain size, on the quality of the machined surface, cutting forces and temperature in the cutting zone, as well as the tool life. In addition, the impact of the feed was considered. The machining process involved side milling of a cuboidal block made of the AISI 316L steel which, due to its specific properties, is widely used in many industries. The tools used in the tests had different WC phase grain size: 0.18, 0.28 and 0.31 μm, respectively, and moreover the middle specimen had also a non-homogeneous structure and an increased content of the Co matrix. The tests proved a significant impact of the tool microstructure on the tool life and the roughness parameters Ra and Rz of the machined surface. The impact of the studied factors on the forces and the temperature in the cutting zone was not as strong, because it did not exceed 20%. The value and the novel character of the paper results from the fact that it concerns a specific case: side milling of the 316L steel with the use of the WC-Co sintered carbide tools, and consequently provides a contribution to solve a practical industrial issue.