Metal-Organic Frameworks (MOF) are the materials that have a three-dimensionally connected and ordered pore structure constructed by the self-organization of the metal ions and cross-linked organic ligands, and a large amount of various gases and organic molecules can be adsorbed in the crystal structure. The studies of gas storage properties of MOF have been researched actively. However, the physical properties of the guest molecules adsorbed in the micropores are not yet resolved. In this paper, the phase transition properties of some organic substances absorbed in IRMOF-1 was reviewed. The heat capacity measurements and X-ray diffraction studies for benzene absorbed IRMOF-1 are described. The results of DSC measurements carried out on the various organic molecules absorbed in IRMOF-1 (dichloromethane, chloroform, carbon tetrachloride, benzene, cyclohexane, chlorobenzene, toluene, n-hexane) with changing the amount of absorbed molecules are also discussed. The melting was observed at a temperature different from the melting point of the bulk. In the case of most organic molecules (dichloromethane, chloroform, benzene, chlorobenzene, toluene, n-hexane), the phase transition was observed at the lower temperature of the bulk melting point. On the other hand, the melting temperature was observed at the higher temperature of the bulk melting point for carbon tetrachloride and cyclohexane. The phase transition mechanism of the substances absorbed in the micropores of HKUST-1 are discussed.