Recently, the demands for miniaturized RF transceiver has been increasing in commercial mobile communication system. In this paper, we propose the active 90 ° phase divider and 90 ° phase combiner for MMIC application. Because of their very large size, conventional passive 90 ° phase divider and combiner such as branch-line coupler can't be integrated on MMIC. Therefore, highly miniaturized 90 ° phase divider and combiner are required for a development of highly integrated MMIC. The active 90 °phase divider and combiner employs the structure of common-emitter and common-collector circuit, respectively. The L, C parallel circuit was connected to emitter of HBT for phase control. And the optimal inductance and capacitance values of L, C parallel circuit was determined to obtain a phase-difference of 90 ° from the two output signals of divider. For a coupling of two signals with a phase difference of 90 ° the L, C parallel circuit was also used for combiner. In this work, we developed active 90 ° phase divider and combiner employing InGaP/GaAs HBT. According to the measured results, it was found that the active 90 ° phase divider and combiner exhibited good RF performances comparable to conventional passive divider and combiner, and they could be integrated on MMIC due to their small sizes. The size of fabricated 12GHz active 90 ° phase divider was about 32% of the conventional passive divider, and 2.4GHz active 90 ° phase combiner was about 2.2% of conventional passive combiner.