This study aimed to implement an unsupervised classification method through the Gaussian mixture model to classify different materials using the scatter diagram of the linear attenuation coefficients acquired from dual-energy micro-CT imaging. This method estimates each cluster's distribution parameters and performs classification based on the posterior probability with a pre-determined cluster number. Our studies on dual-energy images of a phantom showed that the distribution of linear attenuation coefficient of different materials on the scatter diagram has a Gaussian distribution, and clusters can be classified using model-based clustering. The result of this classification method is related to the actual materials in the phantom, where a specific cluster represents each material. This classification method can be potentially used when the clusters are overlapped and the material is separated with high accuracy.