The NTC thermistors based on spinel-type semiconducting electroceramics with compositions Mn1.25Ni0.75Co1.0-xMgxO4 (0 <= x <= 1) were synthesized by solid-state method. The influences of Mg content on the Mn1.25Ni0.75Co1.0-xMgxO4 materials were investigated by laser particle size analyzer, XRD, SEM, EDS and electrical measurements, respectively. The experiment results show that the substituted Mg suppresses the separation of NiO phase. Both the grain size and density decrease as the amount of Mg content in Mn1.25Ni0.75Co1.0-xMgxO4 samples increases. In particular, the substituted Mg leads to a decrease in the resistance drift of the Mn1.25Ni0.75Co1.0-xMgxO4 NTC ceramics after aging test. This indicates that the Mg substituted Mn1.25Ni0.75Co1.0-xMgxO4 NTC ceramics display high electrical stability in comparison with the Mg-free Mn1.25Ni0.75Co1.0O4 ceramics.