One of the greatest challenges of testing the high-temperature mechanical properties of nanocrystalline materials is to find a single-phase material that is stable at test temperature. Tensile tests of different microstructures arising from different heat treatments of Fe-based metallic glass are presented. Most interestingly, a single phase α-Fe microstructure with equiaxed, randomly oriented 15 nm grains is produced through the devitrification of Vitroperm (Fe 73.5Cu 1Nb 3Si 15.5B 7) metallic glass. This microstructure was stable during tensile testing at 600°C, showing a strain rate exponent correlating to grain boundary sliding (m=0.5), but little ductility, and strengths to 1250 MPa. It is suggested that the lack of dislocation activity at the small grain sizes can cause the observed brittle behavior. At temperatures up to 725°C, grain growth occurs, leading to elongations as large as 65% at flow stresses of 250 MPa. This investigation is supported by NSF, Division of Materials Research, grant NSF-DMR-0240144.