Different from distributed baluns, active baluns have group delay variations in the lower bands related to inherent internal capacitances and resistance in transistors. A negative group delay (NGD) circuit is employed as a compensator of group delay variation for an ultra-wideband (UWB) active balun. First, three-cell NGD circuit is inserted into a simple active balun circuit for realizing both group delay compensation and return loss improvement. The simulated results show a group delay variation of 4.8ps and an input return loss of above 11.5dB in the UWB band (3.1-10.6GHz). Then, a pair of one-cell NGD circuits is added to reduce the remaining group delay variation (3.4ps in simulation). The circuit with the NGD circuits was fabricated on an InGaP/GaAs HBT MMIC substrate. The measured results achieved a group delay variation of 7.7ps, a gain variation of 0.5dB, an input return loss of greater than 10dB, and an output return loss of larger than 8.1dB in the UWB band.