Small detection volume, increased analysis speed and reduced cost are the main driving forces for miniaturized lab-on-a-chip systems. Subwavelength holes on opaque metal films provide a unique configuration for miniaturized sensors. Transmitted light through these tiny holes is governed by the electronic resonance on the surface of the metal film. Excitation of surface plasmon-polaritons (SPPs) on the metal-dielectric interface characterizes the resonance condition. The sensitive dependence of the plasmon resonance condition on the dielectric constant of the medium is used for label free sensing applications. In this thesis, we demonstrate a refractive index sensor based on a subwavelength plasmon interferometer using monochromatic light. Very high contrast fringe pattern is generated by the plasmon interferometer that consists of a sub-wavelength slitgroove pair with a small angle between them. The small angle between the groove and the slit provides spatially varying slit-groove distance which generates a highcontrast interference pattern. By interrogating the relative position of interference fringes, one can determine the refractive index of the dielectric medium on the metal surface. The presented plasmon interferometer provides a practical yet sensitive refractive index measurement scheme with very small detection volume.