A clear understanding of the origin of magnetism in metal organic frameworks (MOFs) would provide useful insight for tuning the electromagnetic properties of MOFs and finding new applications. In the present study, first-principles calculations show that the open paramagnetic metal sites in three-dimensional porous magnets M-MOF-74 (M = Ni, Co, Fe, Mn) favor high-spin electronic arrangement. Fe- and Co-MOF-74 exhibit ferromagnetic (FM) features and significantly distinct energy gaps between spin-up and spin-down channels in metastable states. After replacement of the Co center with a Ni ion, the FM feature was exhibited for the stable state since the "extra" valence electron was filled in the spin-down 3d bands to shift the Fermi level to higher energy. In contrast, after removal of one valence electron (i.e., replacement of the Fe center with Mn atoms), the energy gap was significantly enlarged and an antiferromagnetic (AFM) feature will be discerned.