When a transverse magnetic-polarized beam illuminates normally on an Ag grating mask with sub-wavelength slits, the surface plasmon (SP) interference patterns are formed on the Ag surface. According to the dispersion relationship of SPs, the wavelength of SPs (λsp) is tunable by altering the refractive index of the photoresist or the illumination wavelength. Various λsp form interference patterns with different size. The interference patterns are tunable using the same mask with period microns in size, which can save the cost of fabricating a different mask. This method will have potential applications to nanolithography.