Abstract: This paper studies the propagation properties of Quasi-longitudinal leaky surface acoustic wave(QLLSAW) on Y-rotated cut ST-quartz substrates. The phase velocity of QLLSAW on the ST-quartz substrate is about twice that of normal SAW, and in some directions even up to 7000m/s. Both theoretical and experimental results show that QLLSAW propagating along some directions are with small power flow deviation and small temperature coefficient, such as along the direction of Euler angle(0°,155.25°,42°),the phase velocity is 6153m/s, temperature coefficient is 11ppm/℃ and the energy flow of QLLSAW is along the direction of wave vector. When liquid was put on the surface of the substrates, QLLSAW had little absorption by liquid, which proved that the direction of particle vibration is parallel with the surface, and demonstrates that the wave is of the properties of longitudinal wave.