Abstract: A precision micro-current source is designed for the test and calibration of micro-ammeters. It is based on the digitally controlled structure of microcontroller system and uses the circuit design method for the micro-current situation to achieve a micro-current output with the range of 0~200.0nA. The temperature drift analysis and the software calibration method for the V/I convert circuit are discussed. The test shows that the calibrated micro-current source can work steadily with a high precision and the error less than ±0.01%FS.