Sidewall roughness-caused optical loss of waveguides is one of the critical limitations to the proliferation of the silicon photonic integrated circuits in fiber-optic communications and optical interconnects in computers, so it is imperative to investigate the distribution characteristics of sidewall roughness and its impact upon the optical losses. In this article, we investigated the distribution properties of waveguide sidewall roughness (SWR) with the analysis for the three-dimensional (3-D) SWR of dielectric waveguides, and, then the accurate SWR measurements for silicon-on-insulator (SOI) waveguide were carried out with confocal laser scanning microscopy (CLSM). Further, we composed a theoretical/experimental combinative model of the SWR-caused optical propagation loss. Consequently, with the systematic simulations for the characteristics of optical propagation loss of SOI waveguides, the two critical points were found: (i) the sidewall roughness-caused optical loss was synchronously dependent on the correlation length and the waveguide width in addition to the SWR and (ii) the theoretical upper limit of the correlation length was the bottleneck to compressing the roughness-induced optical loss. The simulation results for the optical loss characteristics, including the differences between the TE and TM modes, were in accord with the experimental data published in the literature. The above research outcomes are very sustainable to the selection of coatings before/after the SOI waveguide fabrication.