In a magnetically resonant wireless power transmission system, a DC-DC power transmission efficiency (ηTOTAL) at an inherent resonant frequency (fRES) is degraded when the distance between a transmitter (TX) coil and a receiver (RX) coil is short, because the frequency dependence of ηTOTAL has two peaks. In order to solve the efficiency degradation, a zero phase difference capacitance control (ZPDCC) is proposed, which is suitable for the integration to LSI's. In ZPDCC, either of the two peaks is shifted to fRES and ηTOTAL is increased by tuning the capacitance (C) of the resonator in TX and RX to keep the zero phase difference (θ = 0) between the voltage and the current in TX at Δθ / ΔC > 0. Both TX and RX circuits are fabricated in a 3.3V, 180nm CMOS. By introducing ZPDCC, the measured ηTOTAL at fRES of 13.56MHz increases 1.7 times from 16% to 27% at the distance of 2.5mm between the TX and RX coils with a diameter of 40mm.