Aluminum nitride (AlN) is a promising Acoustic Emission (AE) sensor element for high-temperature environments such as gas turbines and other plants because AlN maintains its piezoelectricity up to 1200°C. Highly c-axis-oriented AlN thin-film sensor elements were prepared on silicon single crystals by rf magnetron sputtering. Both ordinary-temperature AE sensors and high-temperature AE sensors have been developed using these elements. In this paper, to study effects of d33 and thickness of AlN elements on sensor sensitivity, AlN elements with d33 from 2 to 7 pm/V and thickness from 3 to 9 /m were prepared. It is confirmed that the AE sensor sensitivity increased with d33 and thickness of AlN elements. The sensitivity of the high-temperature AE sensor was also improved by a design of the sensor structure. The sensor characteristics were evaluated at elevated temperatures from 200 to 600°C. It was confirmed that the AE sensor works well at 600°C and does not deteriorate.