Abstract: The hot deformation behavior of 5Cr9Si3 valve steel was investigated by hot working simulation test at different hot working parameters. When the deformation temperature increases from 850 to 900℃ or from 1000 to 1100℃, the peak stress decreases; but when the deformation temperature increases in the range of 900 to 1000℃, the peak stress rises. Further microstructural and phase transformation analyses show that a transformation from α-ferrite to austenite occurring within the deformation temperature range from 900℃ to 1000℃ causes austenite phase transformation strengthening, inducing the increasing of hot deformation resistance. Also, with the increasing of deformation temperature, the dissolution of carbides leads to solution strengthening for the matrix. Thus, phase transformation strengthening and solution strengthening at elevated temperatures are the main reason for the abnormal variation of flow stress for 5Cr9Si3 valve steel.