This paper presents development of FDTD-based CAD simulator using MATLAB graphic user interface feature for solving the coaxial applicator problem. The capability of the CAD is to visualize the 2D and 3D electromagnetic field and bio-heat distribution. The CAD is competitive with commercial simulator in term of learning curve and simplicity in the program algorithm. In this paper, the creation steps of the CAD are described in detail. In addition, the comparison result between the CAD and commercial simulator are intensively discussed and analyzed. Some of the improvements of this CAD are also presented.