Resonant grating waveguide (RGW) biosensor has been widely investigated in recent years. The system has several advantages such as minimizing, label-free, high throughput, real time monitoring and so on. The sensitivity of RGW biosensor was theoretically analyzed for different resonance wavelength. Sensitivity to the refractive index and thickness of the sample was analyzed using rigorous coupled wave analysis method. Results indicate that the sensitivity to refractive index is constant for a certain resonance wavelength as the thickness of sample does not change. The sensitivity to refractive index is enhanced with the increase of resonance wavelength. RGW with 1 250 nm resonance wavelength is approximately 4 times as sensitive as RGW with 830 nm resonance wavelength. The sensitivity to thickness drastically decreased at first, and then achieved zero for a certain resonance wavelength as the refractive index of sample is invariable. It was showed that it is limited to test sample thickness for every RGW. RGW with 1 250 nm resonance wavelength is approximately 2 times the measure range as RGW with 830 nm resonance wavelength. The sensitivity to thickness is enhanced with the increase of resonance wavelength. RGW with 1 250 nm resonance wavelength is approximately 3 times as sensitive as RGW with 830 nm resonance wavelength. The above results reveal that the sensitivity to both sample refractive index and sample thickness is enhanced with the increase of resonance wavelength. And the capability of testing sample thickness improves with longer resonance wavelength. The results provided the theory basis for resonance wavelength choice of RGW biosensor.