A novel CMOS frequency doubler circuit is presented in this paper. A common source transistor pair biased at threshold is used to rectify the input signal in both the positive and negative cycles. The rectified signals are then subtracted to generate a double frequency signal. Measurement results show that there is more than 20 dB fundamental rejection with the input power level ranging from -20 dBm to -10.3 dBm. The 3rd and 4th harmonic rejections are above 20 dB with input power up to -10 dBm without any on-chip or off-chip filtering.