The potential energy surfaces corresponding to the photolytic reactions of 1,2-dimethyl-cydopentene, 3,4-dimethyl-silacyclopent-3ene, and 3,4-dimethyl-germacyclopent-3-ene were investigated by employing the CAS(6,6)/6-311G(d) and MP2-CAS-(6,6)/6-311++G(3df,3pd)//CAS-(6,6)/6-3-1G(d) methods. Also, six kinds of substituted germacydopent-3ene were used as model reactants by way of the CASSCF and MP2-CAS methods to study their photolytic mechanisms. The theoretical findings indicate that the photolysis of the above reactants all adopt the same reaction path as follows: reactant Franck Condon region > conical intersection germylene and 1,3-butadiene. However, the theoretical results demonstrate that no photolysis (1(2c *a4)) can be observed in the 1,2-dimethyl-cydopentene system. Above all, the theoretical investigations strongly suggest that both steric effects, originating from the bulky substituents, and the atomic radius of the group 14 element (C, Si, and Ge) play a crucial role in determining the cis/transselectivity of the conformation of 1,3-butadiene during their photolytic reactions.