Abstract: A Resistive Trans-impedance Amplifier(RTIA) readout circuit for Uncooled Focal Plane Array (UFPA) using active resistor was proposed in this paper. By using a sub-threshold MOSFET as a 1011 and above feedback resistor, a high gain current amplifier could be realized by common source structure which consisted of two transistors. The simple three transistors could be easily integrated under pixel and it had good impedance matching with pyroelectric infrared detector. Compared with traditional RTIAs which use special high-resistance materials as feedback resistor, the novel RTIA was low cost because no additional materials and processes were needed. The circuit was successfully manufactured by 0.5 um standard CMOS process of CSMC foundry. 40 dB gain and 3 V output swing were realized by this design and the gain and bandwidth of the chip kept stable during high and low temperature tests. So it can be used for large arrays pyroelectric detectors such as Lead Zirconate Titanate(PZT) and Barium Strontium Titanate (BST), etc.