Based on the characteristics of evanescent wave moire fringes generated from subwavelength gratings, we numerically investigated the contrast of moire fringes for two gratings with and without a silver slab. The effect of the gratings' geometrical parameters on the contrast of moire fringes is analyzed in detail by using the finite-difference time domain method. Numerical results show that great enhancement of the contrast of evanescent wave moire fringes could be achieved by surface plasmon excitations on a silver slab. These evanescent wave moire fringes with high contrast will have potential applications in the nanofabrication field. (C) 2008 Optical Society of America.