Many polymeric materials are weak of direction in the mechanical properties, and which is one of the most important specifications required for structural applications. Therefore in this work, it was prepared two groups of composites nanomaterials, both of which contain the same matrix material, but they differ in terms of reinforcement materials. One of these groups consists of a poly ((methyl methacrylate (PMMA)) resin as a matrix material, which reinforced with magnesia (MgO) nano powder, with the average particles diameter (52.54) nm, which was prepared according to the ratios (PMMA: X% MgO) where (X = 0 , 0.5, 1 and 1.5) of the volume fraction ratio of nanoparticles in composites of material prepared, while the second group reinforced with alumina (AL2O3) nano powder, with a mean particles diameter (53.60) nm, according to the ratio (PMMA: X% AL2O3) where (X = 0, 0.5, 1 and 1.5) of the volume fraction of nanoparticles in composites of the prepared materials. And through this work, effect of the addition nanoparticles powders, on the some mechanical properties of the prepared samples, has been studied. The results showed that the mechanical properties of (Tensile strength, modulus of elasticity, compressive strength and hardness) increase with the increase of the volume fraction ratio of the alumina and magnesia nanoparticle content in composites material. As well as the results showed, that the nanocomposite samples (PMMA: X% Al2O3) have mechanical properties higher than their counterparts of nanocomposites samples, which are reinforced with magnesia nanoparticle at the same ratio.