Sensitive thin film layers of SnO2, SiO2 and SiO2-SnO2 were deposited on a SAW resonator using sol-gel method and spin coating techniques. Their ammonia-sensing performance operated at room temperature was characterized and their sensing mechanisms were comprehensively studied. When exposed to ammonia, the sensors made of SnO2 and SiO2-SnO2 films exhibit positive frequency shifts, whereas the SiO2 film sensors exhibit a negative frequency shift. The positive frequency shift is related to the dehydration and condensation of hydroxyl groups, which make the films stiffer and lighter. The negative frequency shift is mainly caused by the increase of mass loading due to the adsorption of ammonia. The gas sensor based on SiO2-SnO2 film shows a positive frequency shift of 631 Hz when it is exposed to ammonia with a low concentration of 3 ppm, and it also shows good repeatability and stability, as well as a good selectivity to ammonia compared with gases of C6H14, C2H5OH, C3H6O, CO, H2, NO2, and CH4.