A reversible high-efficiency amplifier/rectifier circuit using one transistor has been proposed. For the gate side of the transistor, an input circuit for the amplifier and a gate adjusting circuit for the rectifier are prepared. By switching the circuits, the operation mode is changed. For the drain side of the transistor, a harmonic treatment circuit can be shared by both operations. The harmonic treatment induces high-efficiency operation not only for the amplifier, but also for the rectifier. To verify both operations, a GaAs pHEMT amplifier and a rectifier which varied only the gate-side circuit were designed and fabricated at 5.8 GHz. The fabricated amplifier exhibited a maximum drain efficiency of 71%, and the fabricated rectifier exhibited an RF-to-DC conversion efficiency of 68%.