Abstract: This paper introduces a wireless sensor based upon a passive surface acoustic wave (SAW) device. As the sensing element, an ID-tag and several reflectors are designed on the SAW device. The interrogate system emit a pulse of radio frequency electromagnetic wave which excites a surfaCe acoustic wave transmitting along the surface of the SAW device. The transit time of the acoustic wave is affected by the parameter of the SAW device. Thus when the reflected acoustic wave excites electromagnetic wave through ID-tag and received by the interrogate system, the parameter to be sensed can be get from analyzing the transit time. Because many kinds of physical/cliemical sensors can be developed based upon the SAW device, this wireless interrogation sensing system can be widely used in many applications.