Abstract: The exist style of niobium in 0.25% Si non-oriented silicon steel and the interaction mechanism, and the effect of Nb on microstructure and electromagnetic properties in the finished steel are studied. The results show that, for present work, the solution and precipitation of the niobium inclusions and the traditional inclusions can be improved by adding 0.008 3% niobium into the liquid steel. Thus, the average size of inclusions will obviously decrease from 152 nm to 64 nm,and the number of which will sharply increase from 1.7 x 10⁷/mm³ to 8.4 X 10⁷/mm³. Among of which,the main niobium contained is Nb (C,N) ,with size being hundreds of nanometer grades. They can pin the grain boundary and decrease the diffusion rate of grain boundary significantly, and then resulting the occurrence of the refinement grain size segregating and the island shape grains. Finally,the iron loss and magnetic induction of the finished steel are obviously deteriorated by 0.7 W/kg and 0. 03T,respectively. More over, the deterioration of iron loss and magnetic induction in steel will increase monotonically with increasing niobium content. The Nb inclusions lead to increase of iron loss P_15/50 by deterioration of hysteresis loss.