In this paper, we have proposed a photonic crystal waveguide with a ring-shaped structure of holes for gas sensing applications. The behaviour of the sensor is analysed by using the 3D finite difference time domain (FDTD) method. The proposed structure is designed by etching ring-shaped holes in a silicon layer and by deliberately introducing a row of holes in the waveguide (line-defect). In order to improve the sensitivity, ring-shaped holes are over-etched by a finite depth in the underneath oxide layer. The transmission spectral characteristics and the sensitivity are analysed by varying the defect-radius and the etch-depth. The proposed structure shows a 0.05 nm shift in the cut-off wavelength, corresponding to a refractive index change of 10-4.