A droplet is manipulated by using a surface acoustic wave (SAW). If a sensor is fabricated on a SAW propagating surface, a digital microfluidic system is realized. Our proposed microfluidic system consists of a sensor plate/matching layer/piezoelectric substrate. Droplets are manipulated, mixed and measured on the sensor plate. In this paper, we discuss an acoustic streaming which is caused from longitudinal wave radiation on a micro-laboratory. The acoustic streaming on the three-layer structure was observed in the water tank and compared with it on a 128YX-LiNbO₃. The results indicate that the streaming on the three-layer structure is difference with it on the 128YX-LiNbO₃.