A type of metallic double nanoslit with different widths is proposed to investigate Young's interference mediated by surface plasmon polaritons (SPPs). Numerical calculations show that the Young's interference order could be shifted readily by adjusting the width difference between two slits. Further calculations indicate that the interference order shift related to the additional phase retardation is caused by the distinct surface plasmon mode in two slits. Since the surface plasmon mode in a nanoslit is extremely sensitive to the incident wavelength, it suggests a potential way of ultrahigh resolution spectral analysis via measuring the shift of Young's interference. (C) 2007 Optical Society of America.