With the containerless electrostatic levitation method, the undercooling level and nucleation solidification mechanism of niobium samples of 99.7% and 99.95% in purity were comprehensively studied. The classical nucleation theory was used to measure and calculate the thermophysical and thermodynamic characteristics associated with the nucleation and solidification procedure. The measurement results show that the maximum undercooling of experimental samples is 455.7 K, while the hyper cooling limit of the niobium is derived at 739 K. The most probable nucleation undercooling, pre-exponential factor, nucleation activation energy, solid/liquid interface free energy, and the critical nucleus radius are determined. The dendrite growth velocity has a powerful relationship with undercooling, and the dendritic growth velocity of liquid niobium reaches 42.1 m/s at the undercooling of 454 K. The effect of the anisotropy of surface energy is taken into account, and the forecast results display excellent consistency with the experimental ones.