The surface plasmon resonance gas sensor is presented for refractive index detection using nano-cavity antenna array. The gas sensor monitors the changes of the refractive index by measuring the spectral shift of the resonance dip, for modulating the wavelength of incident light. It is demonstrated that minute changes in the refractive index of a medium close to the surface of a metal film, owing to a shift in the resonance dip of the wavelength, can be detected. The average detection sensitivity is about 3200 nm/RIU （refractive index units）, which is more than twice that of a metal grating-based gas sensor. The reflectivity of the surface plasmon resonance dip is only - 0.03%, and the full widths at half maximum （FWHMs） of bandwidth of the angle and wavelength are - 0.20° and 4.71 nm, respectively.
