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 1 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.