In this study, the effect of Mg substitution on the structural, magnetic and electrical properties of La 0.75 Sr 0.25-x MgxMnO 3 (nominal composition) are investigated by XRD, Ac susceptibility and electrical resistivity measurements. The results show that by increasing Mg doping level, the paramagnetic-ferromagnetic and metal- insulator transition temperatures decreases and reentrant spin glass observed in heavily doped samples. The reason for decreasing transition temperatures with increasing Mg concentration is that Mg ion substitutes Mn site and not the La site, thus the long range FM order was destroyed by the Mg randomly occupying Mn site, which leads to the suppression of double exchange interaction in the Mn3+-O- Mn4+etwork. In this study, the effect of Mg substitution on the structural, magnetic and electrical properties of La 0.75 Sr 0.25-x MgxMnO 3 (nominal composition) are investigated by XRD, Ac susceptibility and electrical resistivity measurements. The results show that by increasing Mg doping level, the paramagnetic-ferromagnetic and metal- insulator transition temperatures decreases and reentrant spin glass observed in heavily doped samples. The reason for decreasing transition temperatures with increasing Mg concentration is that Mg ion substitutes Mn site and not the La site, thus the long range FM order was destroyed by the Mg randomly occupying Mn site, which leads to the suppression of double exchange interaction in the Mn3+-O- Mn4+etwork.