This paper presents a CMOS voltage controlled ring oscillator (VCO) with temperature compensation circuit suitable for low-cost and low-power MEMS gas sensor. This compensated ring oscillator is dedicated to Chopper Stabilized CMOS Amplifier (CHS-A). To operate at low frequency, a control voltage generated by a CMOS bandgap reference (BGR) is described and the measurement results of the fabricated chips are presented. The output voltage of the reference is set by resistive subdivision. In order to achieve small area and low power consumption, n-well resistors are used. This design features a reference voltage of 1V. The chip is fabricated in AMS 0.35 micrometer CMOS process with an area of 0.032 mm2. Operating at 1.25 V, the output frequency is within 200±10 kHz over the temperature range of -25deg C to 80deg C with power consumption of 810 muW. Index terms: Voltage-controlled oscillator, gas sensor, chopper modulation, CMOS, bandgap, temperature compensation, lowpower.