A wide band directional coupler comprises a first transmission line, a second transmission line and at least one inductor, wherein the first transmission line and the second transmission line are adjacent each other, and the inductor connects to the second transmission. As a RF signal passes through the first transmission line, the second transmission line will couple with the RF signal in the first transmission line to generate a coupling signal. The inductor is connected to the second transmission line to improve the coupling factor and/or the insertion loss of the wide band directional coupler at high frequency band.