<p>In thermistor materials, the material constant (B) and resistivity (<i>ρ</i>) of manganese-spinel NTC (Negative Temperature Coefficient) thermistors exhibit a clear correlation: changes in one parameter typically align with changes in the other, with few exceptions. This investigation centers on the system of Ni_0.23Mn_{1.49-<i>x</i>}Fe_{0.039+<i>x</i>}Co_1.24O₄ (<i>x</i> = 0, 0.2, 0.3, 0.33, 0.39, and 0.45) for which, the traditional solid sintering method was used to adjust the Mn/Fe ratio. This study delves into how these varying Mn/Fe ratios affect the electrical characteristics of the thermistor ceramics. As <i>x</i> increases—indicating higher Fe and lower Mn content—room temperature resistivity (<i>ρ</i>₂₅) gradually rises, while the material constant (<i>B</i>_<i>25/50</i>) declines, halting its decrease at <i>x</i> = 0.45. This trend is linked to a decrease in both carrier mobility and carrier concentration, explaining the inverse relationship between resistivity and material constant. This study offers some insights into regulating material constant and resistivity of NTC thermistors.</p>