The EIS of rebars in concrete without chloride ion and with 0. 8mol/L chloride ion were measured. The effects of time and composition of concrete on the characteristics of EIS of rebars were studied. When a straight line, which meant diffusion control, appeared on the EIS, the rebar in concrecte began to corrode. The characteristics of EIS of rebars in concrete were different from those of rebars in simulated pore solutions. There were capaci- tive loops, which indicated the existence of sedimentary film of Ca(OH),, in high frequency range. The corresponding equivalent circuits were proposed respectively, which could inter- pret the corrosion mechanism of rebnrs in concrete.