Aluminum nanoparticles with an average diameter of about 48 nm are compressed in a cemented-carbide mold under different pressures to produce nanocrystalline aluminum by the hot-pressing technology in a high vacuum condition. The X-ray diffraction and the positron annihilation lifetime spectroscopy (PALS) are used to characterize the microscopic structures of nanocrystalline aluminum. The PALS experimental results indicate that there are three types of defects in nanocrystalline aluminum, i.e., vacancy-like defects, vacancy clusters, and microvoids, which are corresponding to three lifetime components of positrons. The pressure for compaction has a great influence on the positron annihilating behavior. The vacancy clusters transform into the vacancy-like defects with increasing the pressure when it is below 0.39 GPa. The three types of defects tend to be rapidly eliminated in a pressure range from 0.39 GPa to 0.72 GPa. When pressure is above 0.72 GPa, the defects are further eliminated in nanocrystalline aluminum. The density and microhardness of nanocrystalline aluminum increase significantly with increasing the pressure for compaction.