We present a micromachined gas sensor which can detect dilute NH3 gas at ppb (parts per billion) concentration based on chemoresistive effect of semiconducting tungsten trioxide (WO3). The sensor structure consists of a poly-Si diaphragm-heater suspended on four tiny beams, which are connected to a frame with overall chip size of 1ױװ.3 mm3. Thanks to the diaphragm-beam structure, the power loss due to thermal conduction effect decreases and uniformed temperature on whole diaphragm is obtained. The sensor has been fabricated and tested. Thermal efficiency of the heater was measured to be 10 oC/mW. The optimal working temperature was found to be around 300 oC, and the sensor could detect NH3 gas at ppb concentration in a background of wet air. The nano-gap effect of the interdigitated electrodes in improvement of the sensitivity was also confirmed. Our estimation based on the measurement results and system noise showed a limit-of-detection of NH3 gas could be down to 2.5 ppb concentration.