Surface acoustic wave (SAW) devices have been shown to be suitable for many sensor applications. One of these applications is pressure sensor. In this study we investigate the performance of SAW pressure sensors formed with ZnO/Si(001) structure. The pressure sensitivities of Rayleigh mode as well as the Sezawa mode are studied as a function of normalized thickness (kh = 2pihZnO/lambda). The experimental results show an opposite strain effect in the ZnO layer and Si substrate. A theoretical approach based on the perturbation method has been developed for the evaluation of pressure sensitivity in the Sezawa mode. Experimental and theoretical results obtained for the ZnO/Si SAW sensor prepared with kh = 1.18 are in good agreement. For kh < or = 1.2, the ZnO contribution to the sensor sensitivity can be neglected in the Sezawa mode in which ZnO acts mainly as an electromechanical conversion layer.