This paper propose highly miniaturized active $90^{circ}C$ phase difference power divider and combiner for application to wireless communication system. The conventional passive $90^{circ}C$ power divider and combiner cannot be integrated on MMIC because of their very large circuit size. Therefore, the highly miniaturized active $90^{circ}C$ phase difference power divider and combiner are required for a development of highly integrated MMIC. In this paper, the highly miniaturized active $90^{circ}C$ phase difference power divider and combiner employing InGaAs/GaAs HBT were designed, fabricated on GaAs substrate. According to the results, the circuit size of fabricated active $90^{circ}C$ phase difference power divider and combiner were $1.67{ imes}0.87$ mm and $2.42{ imes}1.05$ mm, respectively, which were 31.6% and 2.2% of the size of conventional passive branch-line coupler. The output gain division characteristic of proposed divider circuit showed 8.4 dB and 7.9 dB respectively, and output phase difference characteristic showed $-89.3^{circ}C$. The output gain coupling characteristic of proposed combiner circuit showed 9.4 dB and 10.5 dB respectively, and output phase difference characteristic showed $-92.6^{circ}C$. The highly miniaturized active $90^{circ}C$ phase difference power divider and combiner exhibited good RF performances compared with the conventional passive branch-line coupler.