A chemical and biological sensor based on a free-space waveguide resonant grating optical filter has been developed. Different front the conventional surface plasmon resonance sensors and the conventional waveguide mode sensors, which require either prism coupling or grating coupling, the proposed free-space optical sensor device does not require special separate coupling, The inherent 100% coupling efficiency at resonance can significantly boost probe efficiency. Both Simulation and experimental results have demonstrated that the sensor could deliver a resolution of better than 0.001-0.0001 for refractive index sensing, which is enough for detecting various chemical and biological materials. More importantly, under an angular detection scheme, the proposed waveguide resonant grating sensor could be one order's more sensitive than the conventional Surface plasmon resonance sensor. (c) 2005 American Vacuum Society..