This study presents a novel gas sensor in the form of a micro-machined resonator and its readout circuit. The resonator has the structure of clamped-clamped beams with thermal actuation and piezo-resistive sensing that supports a plate capable of being attached with test gas molecules to detect gas concentration. The purpose of this study was to design a new gas sensor readout system for a clamped-clamped beam resonator with thermal actuation and piezo-resistive sensing and also presents a new approach to reject the feed-through noise. The sensor is driven by a DDS module and power amplifier, and then sense the vibrations by piezo-resistivity. The piezo-resistivity is detected by a Wheatstone bridge circuits. The carried signal of modulation is set in Wheatstone bridge circuits. An instrumentation amplifier adjusts the gain to the appropriate amplitude. The circuit with reduction on feed-through noise increases the SNR. Square wave conversion circuit and PFD process the signal and the driver reference signal to detect phase difference. The data of phase difference is counted into a microcontroller dsPIC4011 and then the data being transmitted to the computer by RS232 to a USB adapter. Finally, the whole circuit is implemented by using TSMC 0.35μm 2P4M process and one-step post-processing.