Semiempirical MNDO-PM3 quantum chemical calculations were carried out for the isomers, stable and transition states, of the titled compound (Sp) to reveal the thermal enantiomerization of the photochromic spiropyran. To elucidate the mechanism in DMSO solution, COSMO calculations were carried out. Three possible conformations of photomerocyanines, EE, EZ, and ZE were optimized as planar, whereas ZZ isomer was not planar. Isomerization between EZ and ZZ and ring opening of Sp to ZZ were successfully calculated, while the other processes were prohibited. The mechanism for enantiomerization of (RS)-Sp was explained by a series of (R)-Sp to ZZ(R), ZZ(R) to ZZ(S), and ZZ(S) to (S)-Sp reactions.