Surface acoustic wave (SAW) devices were fabricated on ZnO thin films deposited on Si substrates. Effects of ZnO film thickness on the wave mode and resonant frequency of the SAWs have been investigated. Rayleigh and Sezawa waves were detected, and their resonant frequencies decrease with increase in film thickness. The Sezawa wave has much higher acoustic velocity and larger signal amplitude than those of Rayleigh mode wave. Acoustic streaming for mixing has been realized in piezoelectric thin film SAWs. The Sezawa wave has a much better efficiency in streaming, and thus is very promising for application in microfluidics.