The milling process is widely used industrially and the quality of the obtained milled products should be controlled because it affects their performance in exercise. This work correlates the quality of the machined surfaces with the adopted locating system, the shape deviations of the workpiece datum and the machine tool. In other terms three error sources of machined surfaces were taken into account in the proposed method. An analytical model was set up and implemented through Matlab ® to simulate the quality effects of a milling process. It was applied to two face milling processes characterized by two different locator configurations. It was proved that machine tool volumetric error influences the flatness of the milled surface, while the locator configuration and the datum form deviation affect the orientation of the milled surface, as should be actually.