<p>Internal pressure in solids or liquids is theoretically studied, and cannot be measured directly by experiments. A new heat capacity model is created for general materials in the wide temperature range recently, and shows that heat capacity is dependent on both the temperature and the volume. In this study, we further analyzed the parameters in the heat capacity model, and first established a relation between heat capacity and internal pressure in materials. The potential heat capacity-dependent cohesive energy induces the internal pressure <i>P</i>_c in solids or liquids. The internal pressure in matters reflects the existence of strong intermolecular attractive forces, and can be determined by experimental heat capacity, which is helpful to further understand the nature of the internal pressure or heat capacity in condensed matter physics.</p>