Intra-Body Communication (IBC) is an interesting and emerging communication methodology in recent years. Beneficial from conductive property of human body, IBC treats human body as transmission medium for sending/receiving electrical signal. As a result, interconnected cable and electromagnetic interference can be greatly reduced for devices communicated within human body. These advantages are significant for home health care system, which abundant of interconnected cables are needed. Furthermore, IBC technology also provides an alternative solution to communicate with implanted devices. Currently, pioneer researchers have proposed two general methods for realization of IBC, namely: "Capacitive coupling technique" and "Galvanic coupling/Waveguide type technique". Among the methodologies, the Galvanic technique requires neither return path nor common reference. This feature enables the technology attractive for networking biomedical devices on human body and draws much attention from recent studies. Thus, in this work, a preliminary 3D model of electromagnetic (EM) technique is developed in order to provide insight for electric signal distribution within human body. The development of a mathematical model also facilitates future researches in the aspect of communication theory of IBC, such as: optimizing carrier frequency, identifying channel characteristics, developing suitable modulation/demodulation technique, and etc. In this paper, a mathematical model, which employs a homogeneous cylinder in analogous to a human limb, is obtained by solving the Laplace Equation analytically. The proposed model is simple and preliminary, yet it encourages further development of better model in the next phase and lays a foundation for future research and development of Intra-Body Communication.