Abstract: The forsterite film on oriented silicon steel surface is a thin film with insulating property, which is formed by the reaction of the internal oxidized layer and the MgO coating on the steel surface in the process of high temperature annealing. The high temperature annealing tests of oriented silicon steel with and without MgO coating were carried out. The effects of dew point temperature on the morphology of inner oxide layer and magnesium silicate underlayer on the surface of silicon steel were analyzed by scanning electron microscope (SEM) and electron probe microanalyzer (EPMA). The relationship between the phase and morphology of magnesium silicate bottom layer and the morphology of internal oxide layer was found out, and the main factors affecting the adhesion of magnesium silicate bottom layer were determined. The results show that when the annealing temperature was 1 170 ℃, the dew point temperature was reduced from -10 ℃ to 25 ℃, the bottom layer formed on the coated sample surface, and the diameter of spherical oxide particles below the bottom layer increased from 0.23 μm to 1.49 μm. When the dew point temperature increased to 25 ℃, continuous oxides appearred below the film. The oxide particles under the film could reduce the distribution of the phase with low thermal expansion coefficient on the interface between the film and the matrix, thus reducing the thermal stress on the interface, and finally increasing the adhesion of the film on the steel surface.