Propagation characteristics of laser beams are examined in fractional self-imaging planar waveguides. A potential approach to laser beam combining is proposed by application of planar waveguides with fractional self-imaging. The influences of phase mismatch between laser beams on laser beam addition are discussed. The numerical simulation for two lasers with average power of 300 W shows that the beam addition in fractional self-imaging planar waveguide can improve the parameter M2 of combined beam effectively. The sensitivity of beam addition to the phase mismatch between laser beams has been reduced obviously.