For resonator-coupled wireless power transfer systems, various kinds of coil resonators are studied experimentally. First, it should be noted that those systems are explained by a two-pole bandpass filter model with electromagnetic coupling between resonators. Then transfer loss is estimated explicitly. Three kinds of coil resonators are compared quantitatively, a spiral coil, an edge-wise spiral coil, and a solenoidal coil. Unloaded Q, external Q, and coupling coefficient k are measured by experiments. Among them, the spiral coil is found to be suitable for the wireless power transfer. Simulation result shows good agreement with the experimental result and is supported with the experimental result. As a result, it is confirmed that the power transfer system can be designed by the conventional multi-stage filter theory. And the advantage of spiral coil is shown.