The measurement of plasticity in clay bodies is crucial in order to get products free of defects and with less processing time. However, tests which simulate the behavior of the clay during processing and the mathematical modeling of some of its characteristics, particularly the plasticity, become difficult because many variables are involved and there is no consensus on the choice of method to be used. This study aimed to develop a mathematical model based on compression test to evaluate the plasticity of clays. Three types of clays were studied with different levels of moisture and their indices of plasticity were also characterized by the Atterberg's and Pfefferkorn's methods. The experimental data were well fitted by the theoretical curves for a wide range of clay plasticity. Moreover, it was possible to observe a correlation between effective stress of compression and paste moisture within each group of clay.