The perovskite solid solutions of the Sr1-xMgxFe3+ 1-τFe4+ τO3-y system (x=0.0, 0.1, 0.2, and 0.3) were synthesized in N2 at 1,150℃. X-ray powder diffraction study assured that all the four samples had cubic symmetries(SM-0: 3.865 Å, SM-1: 3.849 Å, SM-2: 3.833 Å, and SM-3: 3.820 Å) and that the lattice volumes decreased steadily from 57.7 Å3 to 55.7 Å3 with x values. The nonstoichiometric chemical formulas were determined by Mohr salt analysis and with the increase of x values the amounts of Fe4+ ion and oxygen were decreased simultaneously. Thermal analysis showed that SM-0 started to lose its oxygen at 450℃ and SM-1, Sm-2, and SM-3 began to lose their oxygen at around 350～400℃. SM-0 showed almost reversible weight change in the cooling process. All the samples exhibited semiconducting behaviors in the temperature range of 10~400℃. Conductivities of the 4 samples were decreased in the order of SM-0, SM-1, SM-2, and SM-3 at constant temperature. The activation energies of the conductions were in the range of 0.176 eV～0.244 eV.