Uncertainties in the sub-nanometer range, the use of new materials, roughness, and the three-dimensional structures represent main challenges for the metrology of critical dimensions in nanostructures. In this paper, realistic PMMA lines in the 40nm range are generated by full 3D process simulation and the corresponding secondary electron SEM image is obtained by Monte Carlo simulation. Line Edge and Line Width Roughness parameters are extracted from the simulated SEM image and compared with the parameters obtained from the true geometry.