<p>The present study aimed to evaluate the polymer structure, surface roughness, hardness, and porosity of heat-cured acrylic resin modified with two different concentrations of aluminum oxide (Al₂O₃) nanoparticles. A total of 90 specimens were fabricated and divided into three main groups (<i>n</i> = 10) according to the test; each group was further divided into three subgroups based on the loading ratio of Al₂O₃ nanoparticles: heat-cured samples without addition (control group) and heat-cured samples incorporating with Al₂O₃ nanoparticles at 1 and 2 wt%. The polymer structure, surface roughness, surface hardness, and porosity were assessed using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, a profilometer, a Shore D hardness tester, a photomicroscope, and the Archimedes method, respectively. FTIR spectra showed hydrogen bonding between carbonyl groups and Al₂O₃ nanoparticles, indicating enhanced filler-matrix interactions at higher loadings, the addition of 2 wt% Al₂O₃ improved interfacial adhesion but may increase hydrophilicity, representing a trade-off for applications requiring moisture resistance. Notably, a significant decrease in porosity and surface roughness, along with an increase in surface hardness, was observed in heat-cured acrylic resin containing 1 wt%. Conversely, increasing the nanoparticle loading to 2 wt% increased porosity and surface roughness and decreased surface hardness. These laboratory findings demonstrated that Al₂O₃ nanoparticles can modify the surface and structural properties of 3D‑printed denture base resins. In particular, the 1 wt% Al₂O₃ composite showed the best short‑term surface properties.</p>