Body-centric wireless communication, which is accepted as an important part of the fourth generation mobile communications systems, provides a prospective communication solution for implantable devices in personal health care. In this paper, an inhomogeneous human body model was proposed to study the in-body radio channels for wireless implants, and numerical simulations were carried out with the finite-difference time-domain method. Specifically, parameters, such as channel gain and group delay, were studied for different links according to implant locations, namely, from intra-brain to the surface of chest, abdomen, upper arm, and thigh, respectively, over different frequencies (from 5 to 100 MHz). The influences of electrode size and type were also studied at 45 MHz. The results showed that all channels had the maximum gain of more than −12 dB at 5 MHz. Group delay was almost independent of channels. The model was verified by experimental measurements. The results showed that the simulations were in good agreement with the measurements.