The characteristics of surface acoustic wave (SAW) on undoped GaN/sapphire, p-GaN/sapphire and p-GaN/n-GaN/sapphire are investigated. For p GaN grown on sapphire, the Rayleigh wave mode and Sezawa mode are observed with centre frequency of 255 MHz and 460 MHz, and insertion loss of -42 dB are obtained, respectively. In addition, the effect of annealing process is investigated. The sidelobe rejection of the Rayleigh mode and Sezawa mode are improved by 5.5 dB and 10.2 dB respectively after annealing at 800 K in N2 atmosphere. The results show that GaN film with high resistivity, enough thickness and smooth morphology is a very promising material for real RF monolithic integrated filter.