Abstract <p>Molecular dynamics (MD) simulations have been performed to study the uniaxial tensile responses of nanopolycrystalline niobium. Models with different grain sizes were established by using the Voronoi algorithm, and the effects of grain size and system temperature on the mechanical properties of polycrystalline niobium were investigated. The results indicate that grain size has a significant impact on deformation mechanism of nanopolycrystalline niobium. During the deformation process, the number of atoms at grain boundaries rises significantly, while dislocation density gradually decreases. Young’s modulus and yield stress reduced with reduction of grain size, which accords with inverse Hall–Patch formula. Specimens with smaller grain size have more grain boundaries and a larger proportion of chaotic atoms on grain boundaries, which leads to a decrease in mechanical properties. Young’s modulus and yield strength show an inverse relation with increase in system temperature, which is due to the higher temperature enlarge the number of disordered atoms at grain boundaries.</p>