Stone Mastic Asphalt (SMA) mixtures are susceptible to drain down since they contain more bitumen comparing to Hot Mix Asphalt (HMA) mixtures. To prevent this problem, stabilizers are used in SMA mixtures. Stabilizers are costly materials and needed to be manufactured with several processes. In this study, pumice and diatomite materials were evaluated to see whether they could be used in SMA mixtures as stabilizers. The optimum bitumen contents of the mixtures having different amounts of pumice (1, 1.5 and 2%) and diatomite (0.25, 0.50, 0.75 and 1.00%) were determined in accordance with Marshall design method. Then, the drain down characteristics of SMA mixtures at optimum bitumen content were found. Finally, the resistance of the mixtures to moisture-induced damage is analyzed by conducting the Tensile Strength Ratio (TSR) test. The results indicate that there is a high potential of using certain amounts of pumice and diatomite in SMA mixtures as stabilizers.