The ordered intermetallic Mo₅SiB₂ (T2 phase) is a promising elevated－temperature structural material because of its high melting temperature, and excellent resistance to oxidation and creep. T2 phase has a body－centered tetragonal D8₁ structure (I4/mcm),with 20 Mo, 4 Si and 8 B atoms situating in layered arrangements along the c axis.This structure has been regarded as the obstacle to plastic deformation or dislocation activity especially at ambient temperature. However, like most intermetallic compounds, it undergoes a brittle－to－ductile transition (BDT) up to high temperatures, where the increased ductility is companied by microstructure changes. The feature of high－temperature applications makes it necessary to investigate the BDT behavior. The analysis of microstructure, dislocation configuration and fracture surfaces were carried out by compression and three－point bending tests, coupled with XRD, SEM－EDS and TEM methods. The result suggested that BDT happened over a wide temperature range of 1000－1200℃. At the BDT temperature above, multiple slip systems such as the [010](001) and <110>{110} could be activated under an applied stress, resulting in obvious plastic deformation. Since the edge dislocation displayed more activity than screw and mixed ones, the edge dislocation with b=[010] interacted with the <110>－type dislocation via slipping into its plane of {110}. Therefore, the work hardening was caused by the formation of lots of dislocation nodal points during the subsequent compression process. With temperature increasing, the fracture mode underwent a conversion from transgranular cleavage at ambient temperature to a mix of transgranular cleavage and intergranular failure.