In this thesis, a dual-band equal power division coupler with tunable differential phase is designed and implemented for the first time. This coupler uses four control voltage and eight varactors to control differential phase. For all tuning states, this coupler can theoretically realize perfect impedance matching and perfect isolation, and keeping equal power division. Practically, the output differential phase can be tuned from 60 to 120 degree at the dual operating frequency 900 and 1800 MHz in measurement. In this range of output differential phase, the largest insertion loss reads about 0.81 dB, and both reflection coefficient and isolation are less than -15 dB. As for the largest power division reads about 0.536 dB. Measured and simulated results are in good agreement.