Human Body Communication (HBC), which utilizes the human body as communication channel to convey health informatics, is a prospective communication technique for implantable and surface-mounted medical devices. In this article, we have applied Shannon's theorem to derive the channel capacity for the general galvanic coupling HBC channel. The channel characteristics and channel model of general galvanic coupling HBC channel are firstly discussed. Based on this channel model, the channel capacity is derived. By using the water-filling algorithm, the HBC channel capacity is estimated and the suitable frequency bands for low power data transmission in both on-body and in-body HBC channels are found. The results can provide suggestion for better system design.