The partial molar volume (PMV) is the most fundamental thermodynamic quantity in considering the pressure effects on protein stability and function. The PMV is determined by the hydration as well as the geometry of the solute. However, it is difficult to extract the hydration effects from the PMV by the conventional methods. The 3D-RISM theory, which is a modern statistical-mechanical theory of molecular solvation, enables us to analyze the PMV in terms of hydration. In this article, recent applications of the 3D-RISM theory to some subjects concerning the PMV of protein are reviewed. The topics include the volume difference between low- and high-pressure structures, the volume change associated with the helix-coil transition, and the volume change accompanying ligand-protein binding.