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Silicon-on-insulator (SOI) optical waveguides with high electromagnetic field confinement suffer from side-wall roughness, which is responsible for strong scattering inducing propagation loss. A theoretical investigation of the influence of geometry in submicron SOI waveguides on the scattering loss due to side-wall roughness is reported. Scattering loss coefficient is derived for both narrow and flat SOI strip waveguides. It is shown that scattering loss coefficient is significantly increased for narrow waveguides compared with flatter ones. These results show that attention has to be paid to waveguide geometry, as scattering effects are the predominant source of optical losses in strip submicron SOI optical waveguides.