A Rectifying Antenna (Rectenna) is one of the most important components for a wireless power transmission. It has been developed for many applications such as Space Solar Power System (SSPS), Radio Frequency IDentification (RF-ID), and electric vehicle etc. The Rectenna consisting of RF-DC conversion circuits and receiving antennas needs to be designed for high conversion efficiency to achieve efficient power transmission. In this paper, we investigated the RF-DC conversion circuits by the theoretical analysis. And, we propose the theoretical formula for calculating the output DC voltage in giving the input voltage. The result that is calculated by the formula agreed with the simulator's result, so, we confirmed the validity of the formula we derived. The theory and the formula we derived are useful in order to know the basic operation of the RF-DC conversion circuit, and to design the circuit efficiently.