The magnitude of superexchange interaction of the MnO crystal is calculated according to the methods proposed previously. Various mechanisms of superexchange interaction are taken into consideration. Overlaps between neighbouring ions are assumed to be small and the energy of the crystal is developed in powers of S, the overlap integral between manganese and oxygen ions. The superexchange interaction of all mechanisms appears from S4 terms and it is concluded from calculation that the Slater mechanism may play a dominant role in MnO. The numerical value of magnitude of interaction agrees with experiment in the order of magnitude. Several discussions about superexchange interactions of other substances, particularly of MnF3 and LaMnO3, are given.