An experimental set-up for CARS investigation of gas systems has been established and CABS spectrum of nitrobenzene vapor has been measured. In comparison with the CARS spectrum of liquid phase nitrobenzene, a frequency shift of the NO2-symmetric stretching mode(v =1345cm-1) was observed for the vapor phase. Besides, the CARS spectrum line of nitrobenzene vapor was found to be asymmetric and much broader than that of liquid nitrobenzene.