Improvement of the thermal conductivity of encapsulant material using doping filler is an important requirement for electronic device packaging. We proposed a simple method for determining the thermal characteristics of composite material that can help save time, increase research performance, and reduce the cost of buying testing equipment. Based on the theory of Fourier law, a general 3D model is simplified into a 2D model, which can then be applied to calculate the thermal conductivity of the tested sample. The temperature distribution inside the sample is simulated by the finite element method using MATLAB software; this is a simple and useful option for researchers who conduct studies on thermal conduction. In addition, an experimental setup is proposed to help determine the extent of thermal conductivity improvement in a sample with doping filler compared to a bare sample. This method is helpful for research on optoelectronics packaging, which relates to the enhancement of thermal conductivity composite material.