This paper descries a highly sensitive stain sensor using surface wave (SAW) for battery-less wireless sensor network. A SAW strain sensor measuring method by observing a shift of a resonant frequency was fabricated and numerically discussed. The SAW strain sensor having 40MHz resonant frequency was designed and fabricated onto a Quartz crystal substrate by micromachining. Tensile testing was carried out using a specimen made of SS400 as a test piece. Detectable strain of the sensor is below 10-5 orders. Since the resolution of frequency in our measurement system in the test was 100Hz, this result is correct in theoretically. This result shows our sensor sensitivity based on theory will be up to 10-8 orders in the case of the resonant frequency 1GHz and the resolution frequency 1Hz.