Electric field enhancement in single layer multimode guided-mode resonance filters (GMRFs) is presented. The physical basis that a single layer GMRF can be functioned simultaneously as the waveguide and the phase matching element is demonstrated. Multimode resonance occurs as the grating thickness increases, resulting in electric field enhancement of high order leaky modes. The order of the electric field of maximum amplitude indicates the leakage level of the GMRFs. Thus the bigger maximum amplitude of the electric field will generally correspond to the smaller filter linewidth at the same resonance wavelength.