Magnetic properties as a function of film thickness (2-55 nm) for magnetite (Fe3O4) thin films grown on MgO (100) substrates show that the small thickness films (les 5 nm) are ferromagnetic and possess magnetization values which are much greater than that of bulk Fe3O4 . The inverse thickness dependence of the magnetization points to an interface related mechanism. Whereas, our detailed spin polarized density functional theoretical calculations suggests that there is only a marginal enhancement in the local spin moments of Fe-atom in the vicinity of the Fe3O4-MgO interface and it turns out that the noncompensation of spin moments between the two magnetic sublattices of spinel Fe3O4 is a major factor contributing to the enhancement of magnetization. ; PUBLISHED ; The authors would like to gratefully acknowledge the financial support from the Science Foundation of Ireland (SFI) under Contract No. 06/IN.1/I91.