The majority of the population suffers from dental caries, one of the most common chronic diseases. Therefore, restoration of teeth is an urgent need. The materials used in restoration are composites prepared by adding inorganic components to the polymeric matrix. However, failure due to fractures and secondary caries is still the main problem. Therefore, studies are continuing to improve the mechanical properties and water sorption and solubility properties of the composite. In this study, effects of zirconia, which improves the mechanical properties, and hydroxyapatite nanoparticles, which are the components of the tooth, on the mechanical properties of the composite were investigated. According to the literature, amounts of additives were determined as 1 and 2 wt.% for zirconia and 3 and 5 wt.% for hydroxyapatite. Nine different composites were prepared by mixing with hand spatulation method and mortar mill. The flexural strength and modulus, compressive strength, depth of cure, water sorption and solubility properties of the composites were investigated. Samples were characterized by using Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). Zirconia and HA particles significantly improved the flexural and compressive strength of the composites. The highest flexural strength was obtained in the sample containing 5 wt.% hydroxyapatite and 1 wt.% zirconia, with an increase of 58% compared to the control sample. The highest compressive strength was obtained in the sample containing 3 wt.% hydroxyapatite and 2 wt.% zirconia, with an increase of 22% compared to the control sample. Therefore, zirconia and HA nanoparticles have a synergistic effect.