Abstract: A weak current measurement system is developed. It is mainly used in the accurate measurement of weak current in radiation detection. The principle of the measurement system is introduced. The interference protection measures of current-to-voltage (I-V) conversion and amplification unit, the technology of sigma-delta analog-to-digital conversion and the program design of micro-programmed control unit are described. The system communicates with the computer controlled platform by USB interface, and has functions, such as storage, range selection, data display. Its measuring range is from 0.1 pA to 2.1 mA, and resolution is 41/2 bits. The system has been successfully used in the detection of radiation.