Grain-oriented silicon steel (GOSS) is an important functional material used as lamination cores in various transformers. Its magnetic properties are strongly dependent on the sharpness of Goss texture, which is developed during the secondary recrystallization annealing of product. In order to save energy and reduce cut-down operation costs, Nippon steel first lowered the slab-reheating temperature from 1350~1400 ℃ to 1150 ℃ and adopted the nitriding process to form nitride inhibitors before recrysta-llization annealing in 1970s. In this new process, nitriding is the critical process because it controls the size, distribution and volume fraction of nitride precipitates, which then determines the subsequent deve-lopment of Goss texture. Although it is of great importance for good quality control of industrial GOSS product, unfortunately, a quantitative mathematic modeling on nitriding kinetics is still in lack. In this work, nitriding kinetics were both measured experimentally and simulated by modeling. The N contents after various nitriding periods and N concentration gradient across thickness were both measured. It has been found that the N content increases slowly at the beginning of 60 s and then much more rapidly during nitriding. There exists a sharp N concentration gradient within the depth of 0.03 mm to the steel sheet surface, which diminishes after about 0.04 mm depth. With the different assumptions on N-transfer coefficient from gas to the steel matrix, the first mathematic modeling on nitriding kinetics of GOSS has been successfully established and solved numerically. The simulation results suggest that only when the N-transfer coefficient, f, changes with time following the Avrami function, f=A(1-exp(-ktⁿ)), the calculated nitriding kinetics are consistent with the measurements. Such an Avrami-type dependence results from the reduction kinetics of oxide layer on the surface of silicon steel sheet during nitriding, in which both plate-like and spherical oxides were observed at the beginning but most of them became spherical after nitriding.