The wireless power transfer (WPT) system for electric vehicles requires high efficiency, lightweight, compact and be suited to big air gap. The WPT system has low coupling coefficient, which is far different from the traditional transformer. Optimal parameters design approach for resonant converter with S-S compensation for WPT is investigated. 85 kHz and 20 kHz parameters design are introduced in the paper. The validity of the proposed parameters design of 85 kHz and 20 kHz are verified by experimental results obtained from a 10 kW resonant converter prototype with 150 mm air gap. Finally the results of 85 kHz and 20 kHz optimal parameters design experiment are compared.