The hot deformation behaviour and microstructural evolution of a kind of Ni-Mo alloy (Hastelloy B3) were investigated by Gleeble-3500 thermal simulation tester in the temperature range of 950°C~1250°C, and in the strain rate range of 0.01~5 s-1. Electron backscatter diffraction (EBSD) was used to analyze the microstructure evolution. Arrhenius constitutive model of this alloy was developed on the basis of peak stresses. The microstructural evolution of Hastelloy B3 alloy during deformation was observed via an optical microscope. The critical strain of dynamic recrystallization (DRX) of Hastelloy B3 is identified based on the work hardening rate versus flow stress curves. The DRX kinetics for Hastelloy B3 alloy can be represented in the form of Avrami equation. The results show that DRX behaviour tends to occur at high temperatures and low rates of deformation parameters. According to the EBSD analysis of the grain boundary angle and dislocation density of the deformed organisation of the alloy, dislocations tend to accumulate at the original grain boundaries to form low-angle grain boundaries and then evolve into High-angle grain boundaries. Thus, the main DRX mechanism for Hastelloy B3 alloy is identified as discontinuous dynamic recrystallisation (DDRX) dominated by continuous dynamic recrystallisation (CDRX).