The article presents the results of research on the influence of the length of elements made of carbon fiber reinforced plastics (CFRP) and glass fiber reinforced plastics (GFRP) on surface roughness, surface topography, passive forces and cutting torques after circumferential milling with diamond-coated inserts (PCD). The paper also presents the results of research the stiffness of the elements depending on their length. Samples of composite materials were clamped in a vise at the machining center. The length of the element was defined as the unsupported distance between the milled surface and the place of attachment of the composite element. With constant milling parameters, the maximum values and amplitudes of the values of passive forces and cutting torques at variable element lengths were determined. The obtained surface was measured in order to determine the surface roughness parameters and 3D topography. Research have shown that the carbon fiber reinforced plastics is on average one and a half times stiffer than that the glass fiber reinforced plastics. Based on the results obtained, it was found that as the length of the element increases, the passive forces and cutting torques increase and the roughness parameters increase. It has also been shown that for the glass fiber reinforced plastics, above a certain length, the surface roughness clearly deteriorates.