Optical insertion losses in embedded strip waveguides fabricated in LiNbO3by Ti diffusion have been examined at a 0.633‐&mgr;m wavelength, taking into account resultant near‐field patterns. It is confirmed that the main factors influencing optical insertion losses are radiation loss by the deficient refractive‐index change of a waveguide, scattering loss by insufficient diffused residual titanium oxide film on a waveguide, and coupling loss between a waveguide and a fiber. A diffusion condition for making a low‐loss waveguide is presented, although suppression of lateral diffusion is necessary to achieve further insertion‐loss reduction.