Copper nanoparticles with average diameter of about 45 nm were compressed in a high-strength mold under different pressures at 523 K to produce nanocrystalline copper. The X-ray diffraction (XRD)， scanning electron microscopy (SEM)，and physics-property-measurement system (PPMS) were used to study the thermal capacity of nanocrystalline Cu as a function of temperature and the mass density at low temperatures. The experimental results indicate that the thermal capacity at low temperatures increases with decreasing density. The thermal capacity of nanocrystalline Cu is higher than that of the coarsed Cu and the increase ratio reaches maximum at around 10 K. The physical mechanism of the phenomenon is explored in this paper.