Phase changes during austempering of Si-Cr-Mn steels at 300° or 400° have been studied by means of dilatometric, microscopic and X-ray diffraction methods. The austempering was found to proceed in the following three stages. 1st stage: Supercooled austenite begins to transform into ferrite supersaturated with carbon, causing the enrichment of the rejected carbon atoms in the remaining austenite. 2nd stage: Carbide precipitates out within the ferrite and begins to coagulate. 3rd stage: The remaining austenite rich in carbon begins to transform into an aggregate of ferrite and carbide. However, in the austempering of about one hour, the isothermal transformation is interrupted prior to its completion. Due to the stoppage of the transformation in the process of its progression, the remaining austenite containing 1.0∼1.2%C and of up to 5∼20% in the amount is retained without transforming into other structural constituents. In such austempering, silicon in steels delays the progression of the transformation and results in retaining more amount of austenite in the austempered product.