A transmitting coil with a tumbler structure is presented to improve lateral misalignment tolerance of a wireless power transfer (WPT) system with a planar receiver. The tumbler is made of an acrylic ball and soil. A planar spiral receiving coil wound by Litz wire is centered by Nd-Fe-B magnets. A spherical cap transmitting coil centered also by Nd-Fe-B magnets sticks on the inner surface of the acrylic ball. When lateral misalignment occurs, the transmitting and receiving coils can achieve self-align with the action of magnetic force, and thus the transfer efficiency is improved. A WPT system consisting of two coils with 6 cm outer radii resonated at 107 kHz is studied. A concise theoretical analysis based on circuit model is given first, and then experimental data are reported. The positive correlation between magnetic force and tracking range is given by comparison. Especially, when the transfer distance is 1.5 cm, the WPT system has a 75.5% peak efficiency and a 3.52 W load power at zero lateral misalignment and the tracking range is from 0 to 8.5 cm; In the tracking range the WPT system has a 70.0% average efficiency and the load power maintains over 94.7% of the peak load power. With the increase of transfer distance, the improvements of transfer efficiency and the tracking range both decrease gradually, and the efficiency loss within the tracking range also increases gradually.