Composites with the addition of alumina nanofillers show improvement in mechanical properties.Poly(methyl methacrylate), PMMA, was used as a matrix and two different types of nanofillers having extremely different shapes were added into the matrix to form the composites. The reinforcements were based on alumina nanoparticles having either a spherical shape or whiskers with a length to diameter ratio of 100. The influence of the size and shape of the alumina fillers and the loading on the mechanical properties of the prepared composite were studied using nanoindentation measurements and dynamic mechanical analysis. It was observed that both alumina whiskers and spherical alumina nanoparticles added in the PMMA matrix improved the mechanical properties of the composites, but the improvement was significantly higher with reinforcement by alumina whiskers. The concentration of the reinforcing spherical alumina nanoparticles and alumina whiskers in the PMMA matrix varied up to 5 wt. %. The best performance was obtained by the addition of 3 wt. % of alumina whiskers in the PMMA matrix in terms of the mechanical properties of the obtained composite.