The non-equilibrium segregation of Nb at prior austenite grain boundary (PAGB) induced by welding is experimentally investigated by means of atom probe tomography (APT). The Nb concentration at PAGB appeared to be 0.115 at.% for experimental steel containing 0.06 at.% Nb at the simulated welding thermal cycle for 10s from 1320 to 950 degrees C. The critical time t(c) for achieving maximum segregation is calculated to be 565s for the experimental steel. For the first time, the diffusion coefficient of Nb in desegregation process is calculated for the temperature of T-i =950 degrees C using non-equilibrium segregation model and APT experimental result and is confirmed to be 9.23x10(-11) m(2)/s. Based on it, the kinetics of Nb non-equilibrium segregation at the PAGBs in welding process is outlined, and the segregation process and desegregation process are discussed. The Nb segregation at PAGB is also predicted over the t(8/5) range of 3-600s. The critical t(8/5), at which the highest Nb concentration of 0.144 at.% at PAGB is predicted to be achieved, is29s.