Characteristics of cavitation erosion resistance of Fe-20Cr-1Si-1C-xMn(x=0, 5, 15) alloys were investigated by SEM and XRD analysis. The effects on strain-induced transformations were considerably reduced with increasing the amounts of Mn due to twining that occurred at 5, 15Mn alloys, activating cavitation erosion rates(mg/$ extrm{cm}^2$) which varied as 0.055, 0.114 and 0.160mg/$ extrm{cm}^2$ for 0, 5, 15Mn. From the results, it was found that the addition of Mn element in Fe-base alloy provides more cracking sites at twins rather than absorbing strain energies, so accelerates cavitation erosion rates.