This paper presents a power detection based I/Q imbalance self-calibration technique to compensate signal path gain and phase mismatch in a reconfigurable low-IF receiver. The calibration can be carried out at startup to improve the image rejection ratio (IRR) of the IF complex-bandpass filter, and then configuration words are saved. The system employs an analog I/Q calibration module to adjust signal path gain and phase mismatch, a power detector to measure I/Q imbalances, digital circuits to implement the calibration algorithm, and a digital AGC to adjust IF signals within a predetermined level. The self-calibration system is implemented as a part of a multi-mode GNSS (Global Navigation Satellite Systems) receiver. Only one 2-bit ADC in I (or Q) path is needed to sample the analog IF signal. Measurement results show that the calibrated IRR achieves an average of 50dB in different operation modes which gets a 10~20dB improvement. The low-IF GNSS receiver is fabricated in 65nm CMOS process, and the digital part of the self-calibration system is implemented in a FPGA.