We have investigated the photo-chemical reaction from tetra-tert-butylcyclobutadiene (TB-CBD) to tetra-tert-butyltetrahedrane (TB-THD) and its reverse thermo-chemical reaction processes in the ground state by using CASSCF and MRMP2 computational methods. According to our results, the initial step of the photochemical reaction is the show only photo-physical property without any reactions because the final point of the minimum-energy-path (MEP) calculation at the MRMP2//CASSCF level is the S 1 /S 0 conical intersection (ionic like structure), which results in turning back to TB-CBD in the S 0 state. However, on the way to the final point of the MEP, it is possible for some TB-CBD to transit at another S 1 /S 0 conical intersection (tetra radical like structure), which is related to the photoreaction from TB-CBD to TB-THD. On the other hand, two routes from TB-THD to TB-CBD were found in the S 0 state. One is the route via bicyclodiradical transition state. The other is the ionic transition state. In both reaction 3 paths, only one TS is there in contrast to the plural step reaction suggested previously.