Diamond burnishing is a widely used finishing machining that can have a positive effect on both the roughness of cut surfaces and its stress state. This paper is focused on the examination of the theoretical and real roughness of surfaces machined by sliding burnishing. In determining the theoretical roughness, the surface structure created by the pre-burnishing cutting (turning) was also considered. Two different modelling methods were used to obtain theoretical surface roughness data: CAD-modelling and finite element simulation. A method using CAD-based modelling of the machined surface was used to determine the theoretical roughness for both the turning and burnishing processes. However, this previously developed model is not directly applicable to plastic deformation processes such as diamond burnishing, so the principle of the Hertz theory for normal contact of elastic solids was used to calculate the penetration depth of the tool into the workpiece. The 2D FEM simulations were performed in the DEFORM software. To validate the applied modelling methods, real cutting experiments were performed, where the surface roughness values were measured during diamond burnishing experiments with different feed per revolution values. Based on the comparison of both applied modelling methods with real roughness data it can be stated that the theoretical roughness values are well approximated the real data.