Wireless power transfer (WPT) is required for the diffusion of Electric Vehicle (EV) because it makes possible the process of automatically charging EV. Magnetic resonance coupling is a new technology for WPT, which can transfer a large amount of energy with high efficiency in middle distance. In this paper the topology design of WPT for EV by using this technology is investigated. Through theoretical analysis we obtain expressions of optimal efficiency with different topologies based on equivalent circuits and define a condition criterion, which is related with load resistance, transmission distance, resonance frequency and coil size. According to the condition criterion we can design the appropriate topology easily to guarantee optimal transmission efficiency. Finally, simulations and experiments show that the defined condition criterion has high sensitivity and the proposed method of designing topology is effective and feasible.