Tensile specimens of a Zr-2.5 wt% Nb pressure tube alloy were deformed at strain rates (ε) of 3.2×10-4-2.9×10-2 s-1 and test temperatures (T) of 625-800 °C to investigate the nature of stress-strain curves and the optimum condition for superplasticity. The maximum ductility of 1384% was obtained at T = 700°C and intermediate strain rate of 3.4×10-3 s-1. The stress-strain curves were found to exhibit the dominance of flow hardening, especially in the early part of deformation, followed by flow softening. The variation in ductility and flow stress as a function of test condition is explained by the observed microstructural evolution and fracture behavior.