The precipitation behavior of second phase in non-oriented silicon steel during normalization treatment at 900~1000℃ was investigated by means of scanning electron microscope (SEM) observation and energy dispersive spectrometer (EDS), and thermodynamic and kinetic calculations. The result show that the second phases in non-oriented silicon steel are mainly AlN with a small amount of MnS. The critical nucleation radius (d*) of AlN and MnS particles increases with the increase of precipitation temperature for different matrix phases, namely α-phase, γ-phase and (α+γ) two-phase respectively, which accord with three different precipitation nucleation mechanisms i.e. uniform nucleation, grain boundary nucleation and dislocation nucleation. By taking the second phase nucleation behavior at the same temperature as comparison, among others the critical nucleation work of grain boundary nucleation of AlN is the smallest, the relative nucleation rate is the largest, so grain boundary nucleation is easy to occur in (α+γ) two-phase area. Besides, the critical nucleation radius of MnS on the dislocation line is the smallest, the relative nucleation rate is large, and the initial precipitation temperature is low, therefore, the dislocation nucleation is dominated for MnS precipitates in the α-phase matrix.