For polymer coated surface acoustic wave (SAW) sensors, the frequency shift due to organic vapor absorption in gas phase is much higher than the frequency shift calculated by the mass loading effect of organic vapor on polymer film. A semi-empirical equation based on fractional free volume contribution of gas vapor induced swelling effect has been introduced to describe the amplification effect of organic vapor on viscoelastic polymer films. This equation is based on an assumption that the frequency shift caused by film's viscoelastic changes due to vapor absorption is additive and proportional to frequency shift caused by mass loading effect due to vapor absorption. The frequency shift due to mass loading effect of vapor doesn't means that the initial frequency shift of the polymer film is also only due to mass loading effect of the film. But in this semi-empirical equation, the initial frequency shift by viscoelastic effect of the polymer film before vapor absorption hasn't been taken into account in the amplification factor. This discrepancy causes deviation of prediction. The equation based on perturbation theory is derived to calculate the mass loading effect of gas vapor on SAW sensor. Two equations are compared and the amplification factor in the semi-empirical equation is corrected to include the viscoelastic effect. Responses of SAW sensor to different organic vapors are calculated by the equations derived from perturbation theory and the corrected semi-empirical equation and compared with experimental data to evaluate reliability of the equations.