An analog front-end of a fire detector, which is comprised of a smoke detector, a heat detector and an ADC, is proposed. The smoke detector with active pixel structure works by optical detection measuring the amount of IR LED light scattered by smoke particles. It achieves the reduction of the mismatch from process variation, the influence of dark current and the reset noise by correlated double sampling scheme. The heat detector is a voltage divider consisting of a negative temperature coefficient thermistor and two resistors.It outputs the voltage corresponding to ambient temperature.Each output voltage from the two detectors is alternately converted into an 8-bit code by a successive approximation ADC every 1 second. The test chip was fabricated in 0.35-μm CMOS process and tested in optical chamber. It can detect smoke density range from 4 %/m to 25 %/m with ± 1 %/m accuracy and ambient temperature range from 25 °C to 95 °C with ± 1 °C accuracy. Maximum power consumption is 71 μW.