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.