It is generally said that bonding strength of concrete and rebar decreases if the concrete expand due to ASR. On the other hand, it still remains unclear as how the increment of internal crack due to ASR expansion affects the decrease of bonding strength. Therefore, to clarify how the expansion due to ASR affects the bonding strength of concrete and rebar, bond strength test of rebar and observation of the internal crack after bond strength test were performed. As results of the experiment, the bond strength of the round steel bar decreased dramatically at 0.05% of ASR expansion. It may be said that most of the chemical bonding, and frictional bonding component were lost. On the other hand, bond strength of deformed rebar did not show significant decrease at 0.05% of ASR expansion. That considered due to mechanical bonding effect. After that as expansion increase, deformed rebar also show continued decreasing of bond strength.