Using the effective medium theory and the waveguide eigenvalue equation, we design a multimode planar periodic waveguide. When a plane wave illuminates the grating at the designed angle and wavelength, more than one leaky modes are excited coincidentally. Then the reflection efficiency around this designed angle and wavelength is investigated using a rigorous coupled wave analysis formulation and a gap of reflection peaks is found. Electric field distributions reveal that this high reflectivity gap is due to the coupling between these two coincidentally excited leaky modes.