Geometries of 3-Furyl, 3-pyrryl, 3-thienyl fulgides, and substituted 3-furyl fulgides were fully optimized at HF/6-31G and 6-31G* levels. It was Found that the C-isomers of 3-furyl and 3-thienyl fulgides are more stable than the E-isomers, while the E- and C-isomers are isoenergetic in 3-pyrryl fulgide. The activation energy of thermal ring-closing reaction for 3-furyl fulgide was estimated to be 21 kcal/mol. The electron donating groups enhance the bonding character between atoms to form the single bond in the C-isomer.