Abstract: Two kinds of billets containing 0.09% niobium and niobium free were used for this study.The billets had been heated up to different temperatures and then quenched after holding for 30 min.The microstructure of the samples was observed by JEM-2011 transmission microscope.At the same time,the morphology of black material in as-cast microstructure was determined by scanning electron microscope（SEM）.By observing the quantity distribution of precipitates obtained at different soaking temperatures,the effect of niobium addition on solid solution temperature of precipitates in Nb and Nb-free steels were compared and analyzed.The results show that:The grain size of niobium containing oriented silicon steel billet is more uniform and smaller than that of niobium-free billet.Niobium element can hinder grain growth.The complete solid solution temperature of precipitates in Nb-containing oriented silicon steel is between 1 250 ℃ and 1 300 ℃,and the total solid solution temperature of precipitates in steel without Nb is above 1 300 ℃,indicating that Nb can obviously reduce the complete solid solution temperature of precipitates.