Abstract: The theoretical roughness of machined surface of common metal materials can be calculated using the current formula. However, the machined surface of composites may include a variety of machining defects, and their concave or convex defects may not form directly by the cutting of tool edge. So, the current formula for the calculation of theoretical roughness of conventional metal materials is unsuitable for composites. The machined surface texture and roughness of composites reinforced by fiber are closely-related to the cutting direction. Though the composites reinforced by particles is isotropic, their machined surface textures and roughness are mainly controlled and dominated by the hardness and the volume as well as the size of the reinforcing particle. The interface strength between the reinforce and the matrix, the cutting tool and machining parameters affect the machined surface roughness of composites signally.