The efficiency of acousto-optic interaction in single-mode strip silica waveguide is analyzed theoretically by determining the overlap integral between the optical and acoustic field distributions. The results show that there is a good overlap of the optical and SAW fields in the low SAW frequency range. At high acoustic frequencies, the overlap integral decreases with increasing acoustic frequency. At 216 MHz, the maximum of 0.8544 for the overlap integral is obtained, provided the H/Λ equals 0.02. At last, the diffraction efficiencies for acoustic frequency of 216 MHz are calculated as a function of the square root of acoustic power for different acoustic apertures.