The hot compression curves of Nb bulk materials were measured by Gleeble-3500 thermal simulation machine. The flow stress behavior and the deformation microstructures of the materials were analyzed at the temperature range of 350-480 ℃ and the strain rate range of 0.001-10 S_-1. It is shown that the flow stress reduces with the increasing of the deformation temperature and the strain rate. The elongated grains coarsen with the decreasing of the temperature. And dynamic recovery occurs during the hot compression. The soften mechanism also works by the dynamic recovery. Moreover, the constitutive equation of the strain rate affected by the temperature, strain and the hot deformation activation energy was deduced by the help of the modified hyperbolic sine function of Arrhenius and factor of Zener-HolIomon parameters. The simplified relation was adopted to fit the curves of the flow stress versus the strain at different temperature resulting in good effect.