Grain boundary migration in pure electrolytic iron (Fe > 99.98 %) was studied under isothermal conditions at 1050 °C, 1150 °C, 1250 °C and 1350 °C. High-temperature laser scanning confocal microscopy (HT-LSCM) was used to observe the in-situ grain growth of austenite (γ-Fe) on the sample surface. The dependence of the arithmetic mean grain size on time and temperature were considered in a mathematical model according to classic grain growth theory. As no other effects, e.g., pinning by precipitation or impurity-induced solute drag, occur in pure Fe, the grain boundary mobility M was directly determined by fitting the experimental results. The temperature relationship followed an Arrhenius equation with M = 6.79*10−6*exp(-172750R-1T-1) m4J-1s-1. The mobility obtained differed by more than two orders of magnitude from Turnbull's postulation, which agreed with observations in the literature. The results matched published data extrapolated from a recent study on austenite grain growth in multicomponent steels.