In order to analyze and design a controller of wireless power transfer (WPT), modeling of transient response is important. In this research, we propose envelop model of the load voltage for series-series (SS) compensated WPT circuit via magnetic resonance coupling. A state equation and transfer functions are derived from equivalent circuit of SS compensated WPT circuit. Transfer function which express envelope of a load voltage is also derived. The proposed envelope model is verified by experiment. A feedback controller of the load voltage is designed using the proposed envelope model and effectiveness of the control is verified by experiment.