One of the key performance indicators of photonic circuits is the value of optical propagation losses. Among several factors which impact these losses, the sidewall roughness is considered as the primary focus of this work. The optical signal, propagating along the photonic device, scatters on roughness of its element’s surfaces. This decreases the output power, as well as leads to the higher heating and worse transmission characteristics, which in its turn increases the noise ratio and creates undesired phase deviations. Thus, the problem of sidewall roughness simulation (and consequently, losses estimation in microwave photonic circuits caused by it) is relevant at the design stage of the devices. Therefore, a new, highly efficient model of sidewall roughness based on a photolithography simulation and imitational modelling of photoresist exposure is presented. Principles of operation and implementation features of the model are described. Simulation results, obtained using the new roughness model, are demonstrated and an approach on their verification with experimental data is suggested. Additionally, theoretical estimations for the optical losses caused by sidewall roughness in ridge Si waveguides are discussed.