The effect of optical waveguide sidewall roughness (SWR) on the performance of a 1×2 optical power splitter (OPS) using OptiBPM software for the first time is analyzed. A simulation model is presented to predict the relationship between the SWR and output optical power of OPS for different relatively wide ranges of waveguide width. In particular, different values of waveguide widths, i.e., from w=1 to 6 μm, and an average roughness depth ρ from 0 to 0.5 μm are considered. In addition, the corresponding insertion loss and uniformity are determined. The results show that output power decreases significantly as the SWR increases due to the increase of the scattering loss, especially for ρ≥0.3 μm and narrow waveguides (i.e., w≤3 μm) compared with relatively wider ones (i.e., w<3 μm). SWR can greatly increase the scattering loss leading to significant output power loss, which leads to an increase in insertion loss and uniformity. The conclusion drawn from the presented results provide guidelines for evaluation device performance before real fabrication.