Beta-Ti alloys, for example Ti45Nb, are intensively researched in the last years due to their favorable low Young’s modulus, biocompatibility and bio-inertness, which makes them to interesting candidates for implant materials. Due to their low and thus for various applications insufficient mechanical strength, efforts are currently devoted into increasing their strength. A promising way to improve the properties is provided by means of severe plastic deformation (SPD). In this investigation high pressure torsion was used to refine the microstructure of a Ti-45%Nb alloy inducing a minimum grain size of approximately 50 nm. The main focus of the subsequent investigations was devoted to the thermal stability of the microstructure. Isochronal heat-treatments performed for 30 minutes in a temperature range up to 500 °C caused a further increase of hardness with a peak value at 300 °C before the hardness decreased at higher temperatures. Simultaneously, a distinct rise of the Young’s modulus was measured. Also in uniaxial tensile tests the strength increases after annealing in comparison with the SPD-state, and the ductility only slightly decreases. The findings suggest that the combination of severe plastic deformation with subsequent heat treatments provides a feasible way to additionally improve the mechanical properties of SPD-deformed beta-Ti alloys.