In the polyurethane industry, the undesirable dimerization and oligomerization of methylene diphenyl diisocyanate (MDI) are unwanted and problematic phenomena. The relative ratios of these dimers and oligomers have a strong temperature dependence, and they can be formed below and above the melting point of MDI (316.15 K). All possible structures of the isomers and their dimers were optimized, and then their thermodynamic functions were calculated at the M06-2X/6-31(d,p) level of theory. The dimerization of isocyanate groups can result in four-five-, and six-member rings. The four-member ring is the most stable one due to the aromatic electrons in π orbitals of the uretidione ring. The relative stability of the dimers is therefore influenced by the steric effects and the relative positions of isocyanate groups. The most stable dimer is the one which is formed from the reaction of the p,o- and o,p-MDI isomers due to their intrinsic interactions, which were classified, and their impact on dimer stability was discussed.