The effects of Si content on the variation of microstructure and processing window of austempered ductile cast iron were investigated. Four different Si contents between 2.42 and 3.37wt% were used. The influence of silicon on the microstructure and processing window of these materials were studied. Austenitizing was perfotmed at 900℃ for 60min and austempering temperature wewe both 340℃ and 360℃ and austempering time were for 4min upto 119min and for 5min upto 160min respectively. After heat treatment, the evolution of stage I and stage Ⅱ were performed by optical metallography, XRD, hardness, test The results showed that t2 was delayed as Si contents was increased due to the fact that Si retarded the formation of cementite (Fe3C). The high silicon content promoted the stability of the metastable two-phase combination of austenite and ausferrite.