We investigate the sensitivity enhancement of surface plasmon resonance (SPR) sensors through planar metallic film closely coupled to nanogratings. The effects of the thickness of metallic film and grating period on the refractive index sensitivity of the device are analyzed in detail. The refractive index sensitivity of nanograting-based SPR sensors is predicted to be about 540 nm per refractive index unit (RIU) using optimized structural parameters. Furthermore, the grating period can be used as a parameter to adjust the wavelength of resonance reflection. Our study on SPR sensors through planar metallic film closely coupled to nanogratings demonstrates the potential for significant improvement in refractive index sensitivity, since it shows much greater flexibility in terms of tuning the optical parameters of the device.