This study was designed to improve the thermal conductivity of cold curing PMMA acrylic denture base material at different thickness 2, 3 and 4mm and by adding alumina powder Al 2 O 3 and investigate the effect of this additive on some mechanical-physical (hardness, compression and impact) properties. Alumina powder was added to PMMA powder in percentage 4%. 60 specimens were constructed and divided into two groups according to the tests. The thermal conductivity of the samples had been measured using Lee-disk; while the compressive resistivity had been found using Universal machine. Whereas the hardness test was done using the Charpy’s pendulum impact machine. The results show that after the reinforcement with weight 4% percentage of Alumina powder, mechanical properties (hardness and compression) increased while the impact decreased. The results have shown also that the increase in coefficient of thermal conductivity occurred with the addition of Al 2 O 3 powder.