The influence of the ac Stark effect on the line shift and line shape is studied in the three-photon resonance enhanced ionization spectroscopy of COA (II)-I-1(v’)<--X (1) Sigma(v"=0), where v’=1, 2, or 3. Experimental results show that the line shift varies from one vibrational band to another and differs for various lines in the same band, although it is to a lesser degree. This is given an adequate theoretical elucidation. A detailed calculation, based on the ac stark effect with evaluation of the dipole matrix element, gives -0.66, -0.31, and 0.87 cm(-1) for P-11 of the (1-0), (2-0), and (3-0) band, respectively, where the negative values stand for red shift while the positive one for blue shift. These results are very close to the experimental observations, viz, -0.48, -0.13, and 0.77 for P-11 of the (1-0), (2-0), and (3-0) bands, respectively. Furthermore, the line shape is found to be asymmetrically broadened. For the blue-shift line, the red side is much steeper than the blue side, whereas for the red-shift line, the reverse situation appears. This is again explained as due to the ac Stark effect coupled with the spatial inhomogeneity of the laser field. According to this model, a numerical calculation for the line shape is provided which gives fairly good agreement with the experimental observation.