In this paper, a novel compensation technique is proposed to improve the imbalance of a Marchand balun due to the unequal odd- and even-mode phase velocities of the coupled lines. Using this method, the fundamental rejection of the balanced doubler with the Marchand balun can be effectively enhanced. Two single-balanced doublers using the improved Marchand balun are designed, fabricated, and measured to verify the concept in CMOS processes. One doubler for 15-36-GHz possesses -10-dB conversion gain with the 3-dB bandwidth of 82.4% and the fundamental rejection of 33 dB. The other doubler for 95-150 GHz achieves -7.9-dB conversion gain with the 3-dB bandwidth of 45% and the fundamental rejection of 30 dB. With the proposed compensation technique, these frequency doublers feature wide bandwidths and high fundamental rejections.