The coupling properties between two strongly coupled parallel dielectric slab waveguides are investigated using an analytical method based on waveguide theory. Coupling properties between two waveguides which have different geometrical configurations are given. To specify the strongly coupled waveguides, the coupling coefficient used in this paper is defined as the power transfer ratio. As a result of the analysis, it is shown that a complete power transfer between strongly coupled waveguides cannot exist even if the phase velocities are matched, and therefore an uncoupled factor, defined here, is important. When waveguide parameters are chosen properly, the coupling coefficient becomes insensitive to the structure deviation, and therefore the tolerances in the fabrication processes of the coupled waveguides and related devices can be relaxed.