This study has been carried out to investigate the microstructure of austempered high carbon cast steel with the variation of silicon and heat treatment conditions. The results show that an lower ausferritic structure is formed at the low austemepring temperature ($250{sim}300^{circ}C$) and an upper ausferritic structure is formed at the high austemepring temperature ($350{sim}400^{circ}C$). As an austempering temperature increased, the retained austenite volume fraction increased, however hardness decreased. Also, as a silicon content increased, the precipitation of cementite was suppressed, therefore 2nd reaction of autempering transformation was delayed.