We examined guanine damages caused by photoinduced electron transfer (ET) in biomimetic environments using pyrene-modified oligonucleotides and molecular crowders. As a result, guanine damage induced by ET in full-matched DNA was decreased under viscous crowding conditions. On the other hand, behaviors of mismatched DNA revealed that through-space ET between pyrene moiety and the consecutive guanine site was promoted under molecular-crowding conditions.