This paper presents the quantitative analysis for the optimal design of wireless power transmission (WPT) systems based on magnetic resonant coupling (MRC) mechanism. In this paper, the exemplified MRC-based WPT system adopts the series-series topology and one resonant coil, which shows that the energy efficiency and transferred power are both significantly affected by the mutual inductance of the primary-resonant coils and the resonant-secondary coils. In addition, the simulated and experimental results are both provided to illustrate the influence of the mutual inductance on the performance of MRC-based WPT systems regulating the relative displacement among coils. Thus, this quantitative analysis can offer the significant theoretical and experimental basis for the optimal design of MRC-based WPT systems in various application fields.