The calculations on magnetic exchange interaction Of Cl3FeOFeCl32- and the related modeling compounds were performed by using the density functional theory coupling the broken symmetry approach. The calculated results show the absence of a direct Cl3Fe---FeCl3 magnetic coupling and the effect of the terminal Cl ligands on magnetic exchange interaction in [Fe-O-Fe](4+), while the protonation of mu -oxo bridge reduces significantly the magnetic coupling constant J value. On the other hand, the J value is insensitive to variation of the Fe-O-Fe angle; however, the dependence of the J value on the Fe-O distance can be expressed as an exponential function, while the J value keeps a constant to the variation of the O-H distance in the mu -hydroxo and mu -aqua bridges. Molecular orbital interaction is applied to explain the magnetic exchange interaction in mu -oxo bridged iron(III) dimers. The validity of qualitative magneto-structural correlation for the models used is further confirmed by full geometry optimization.