Refractory metal Nb and its alloys are considered as promising materials in fusion reactor, where they are required to withstand a high neutron irradiation, because their excellent high temperature properties such as high temperature strength, good thermal conductivity and compatibility with most liquid metal coolants. The defects are created in atomic displacement cascade from the primary state of damage and subsequent evolution gives rise to important change in their microstructures and engineering properties. However, the evolution and aggregation of induced radiation defects in atomic level cannot be observed by experiment so far. In this work, molecular dynamics (MD) method is used to explore the microstructural formation and evolution of defects from the atomic displacement cascades in bcc-Nb. In the simulation, the energy range of primary knock-on atom (PKA) is chosen 5~50 keV and the simulation temperature 300 K. It is observed that the most of defects in bcc Nb are point defects at different PKA energies. The vacancy cluster rate varies from 17% to 35% and self-interstitial cluster rate varies from 23% to 40%. As the PKA energy increasing, vacancies usually tend to form larger clusters. The self-interstitial atoms form a dumbbell structure along the direction <110>. The 1/2<111> intermittent dislocation loop and <100> vacancy dislocation loop are produced when the PKA energy greater than 30 keV. The quantitative relationship between energy of PKA (E_PKA) and number of survivals Frenkel pairs (N_FP) is fitted by a power function with different parameters at low-energies (5~30 keV) and the high-energies (30~50 keV).