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The response of GaN/sapphire SAW delay line to the focused ultraviolet (UV) illumination was investigated at the SAW frequency 306 MHz and optical wavelength 375 nm. The UV irradiation of an interdigital transducer (IDT) led to 4 to 5 times larger phase change of transmitted signal than the irradiation of the surface acoustic wave (SAW) propagation path between the IDTs at the same UV light spot size of 2.4 mm. The UV-induced phase change has been attributed to the variation in IDT impedance due to photo-capacitive effect. Simulation of the SAW sensor phase response on the basis of this model is in good agreement with experimental results.