An in-situ nanoindentation test using a Berkovich pyramidal diamond indenter was performed to examine the mechano-electrochemical properties of the single crystal iron (100) surface passivated at a constant potential for 1 h in pH 8.4 berate solution. The load-depth curve measured at a maximum load, W-max = 50 mu N and a load speed of 50 mu N s(-1) showed an evidence of plasticity irrespective of the potential at which the passive film was formed. The average hardness of the iron surface obtained from the unloading curve ranged from 2.0 to 2.5 GPa and it increased with increasing potential in the passive region higher than 0.25 V versus SHE, indicating that the passive film contributes to the increase in hardness of the iron surface. Moreover, the indentation at a maximum load, W-max = 200 mu N and a load speed of 20 mu N s(-1) was performed in order to observe the time variation of the shape of the indent with AFM after the indentation. The indent shape for the iron surface cathodically reduced did not change significantly with time after the indentation. In contrast, the indent shape for the iron surface passivated at 1.0 V spread and became obscure with time, which was ascribed to the active dissolution of iron near and under the indenter, followed by the repassivation.