We have observed that a syn-tricyclo[4.2.0.0(2,5)]octane derivative (1) isomerized keeping the single crystalline state to a (Z,Z)-cycloocta-1,5-diene derivative (2) during the X-ray crystallographic analysis. In this paper, we report that an octahydro derivative (5) of I transformed to a (Z,Z)-cycloocta-1,5-diene derivative (6) keeping the single crystalline state. Similarly, irradiation of an acetonitrile solution of 5 with high-pressure mercury lamp (>400 nm) in the presence of a sensitizer 9,10-dicyanoanthracene, gave 6 in good yields via a radical cation mechanism.