Microdevices have been implanted in the body to diagnose diseases and treat functional disorders, such as an artificial sphincter for fecal incontinence. Since these devices are expected to work in the body as long as possible, the energy supply has become increasingly important. Wireless power transfer (WPT) systems are suitable for medically implanted devices. A transcutaneous energy transmission (TET) system, as one of the wireless power transfer systems, is discussed in this article. A TET system comprises two coils: a transmitter coil and a receiver coil. First, a serial-serial resonance circuit is adopted in the TET system by different resonance circuits’ efficiency analysis. In addition, coil features, such as magnetic field intensity, coil outer diameter, coil turns, and coil inner diameter, have been investigated in detail to improve the inductive coupling coefficient, leading to a high-power transmission efficiency. The proposed TET system with the optimal parameters is used in the in vivo experiments. Experiments using an optimized wireless power transfer system show a better performance than previous (before optimization) with a 38% increase in efficiency.