A novel tri-band branch line coupler is proposed for wireless power transmission system.A conventional coupler loaded with quarter-wavelength short coupled lines is adopted to realize the triple passbands.Each passband has a high degree of isolation.The passband center frequencies of the coupler can be adjusted over a wide band by changing the even/odd-mode characteristic impedances of the coupled lines.A planar tri-band coupler located at 0.69/1.0/1.4 GHz is designed and fabricated for verification.Good agreement can be observed between measured results and theoretical expectations.