Abstract: Ab initio calculation of quantum chemistry at HF/6 31G ** and MP2/6 31G ** levels was employed to study the rearrangement reactions of formylketene and benzoylketene. Geometries of the stationary points on the reaction path have been optimized with the Berny energy gradient technique and the transition states were characterized by vibration frequency analysis. The results show that two reactions both proceed via one elementary step and a four membered cyclic structure was formed in the transiti...