Nonstoichiometry($delta$) and the phase stability region of a ferrite spinel (Mg0.29Fe0.71)3-$delta$O4 have been investigated by a coulometric titration method as a function of temperature(T) and oxygen partial pressure(Po2). It has been found that the spinel is thermodynamically stable in the ranges -8.0$leq$log(PO2/atm)$leq$-2.4, -7.0$leq$log(PO2/atm)$leq$-1.7 respectvely at 100$0^{circ}C$. The nonstoichiometry extends over the ranges of -0.004$leq$$delta$$leq$0.007, -0.008$leq$$delta$$leq$0.006, -0.033$leq$$delta$$leq$0.004 at 100$0^{circ}C$, 120$0^{circ}C$, respectvely. The observed PO2-dependence of $delta$ suggests that the majority ionic defects are cation interstitials in the low PO2 region and cation vacancies in the high PO2 region.