The stereochemical aspects of epoxidation of norbornene and its 7-syn-substituted derivatives by performic acid were investigated. Geometry and thermodynamic parameters of transition states and prereactive complexes were computed at the UBHandHLYP/6-31G(d) level of theory. It is shown that the transition states have a pronounced biradical character and a nearly coplanar orientation of the C=C bond and the molecule of performic acid. Transition state analysis revealed that, in the case of the syn-7-hydroxy derivative, the preference for the exo-approach of the oxidant can be explained by the stabilization of transition state with hydrogen bonding. In contrast, a chlorine atom or a methyl group at the 7-syn position facilitated the formation of endo-epoxides due to steric repulsion between the substituent and the oxidant.