Guided wave second-harmonic (SH) generation can not be phase matched automatically by Cerenkov radiation scheme. There is still phase-matching condition to be satisfied in case of the SH generation in radiation mode of the waveguide. The phase matching condition is so critical that all the fundamental wave (the driving source), the generated SH wave, and the free SH wave multiply reflected and transmitted across the boundaries of the waveguide have to keep in phase. This kind of phase matching can be obtained by typical waveguide technique, i.e. to change the boundary condition. With the unsymmetrical slab waveguide scheme, phase matching can be achieved by simply changing the cover material with different index. The formula is given.