A new state parameter that is capable of characterizing the mechanical properties of cement-treated clays with different mixing proportions, curing time, and stress conditions is developed in this study. The development is conducted by combining the effect of stress level based on the critical state theory with the effective void ratio. Based on the experimental data of cement-treated Bangkok clay performed in this study together with those of cement-treated Singapore marine and Ariake clays reported in previous studies, the analyses reveal that the state parameter is proven to be an efficient parameter for characterizing the mechanical properties of cement-treated clays with different base clays under various mixing proportions and curing times tested under various stress conditions. Furthermore, empirical equations to assist the establishment of the effective void ratio on the critical state locus from the plastic limit value of the base clay are suggested based on regression analyses. The state parameter can thus be computed and the engineers can preliminarily estimate the mechanical properties of cement-treated clay in the planning stage.