Abstract Surface acoustic-wave (SAW) oscillators have advantageous features compared with other technologies. The Y-128° LiNbO3 is a well-known, high electromechanical-coupling SAW material, but its usefulness is limited because of its poor temperature-stability property. The AlN films have some excellent characteristics, such as high SAW velocity, stable chemical properties, and high-temperature stability. In this research, different thickness AlN films were sputtered on Y-128° LiNbO3 to be a composite substrate for the SAW oscillator. As AlN film thickness increased, the temperature coefficient of frequency (TCF) value of the oscillator varied from −76.32 ppm/°C to −28.81 ppm/°C giving an improvement in the TCF of 62.25%. The oscillator frequency at 25°C also varied from 40.0909 MHz to 41.6221 MHz, giving an improvement in the oscillator frequency of 3.8%. The composite substrate (AlN/Y-128° LiNbO3) can enhance the oscillator frequency and effectively improve the poor temperature stability for the SAW oscillator.