Thermal decomposition kinetics of dicyclopentadiene-1,8-dione 7 implied an intramolecular competition between α,β- and β,γ-double bond to assist the CO elimination. Experimental thermolysis of 7 in dioxane gave 3a,7a-dihydro-1H-inden-1-one (cis-bicyclo[4.3.0]nona-2,4,7-triene-9-one), CO gas, and a very small amount of indanone. This result suggested β,γ-double bond favored the extrusion of CO gas. Calculations of several density functional theory (DFT) levels of theory and CBS-QB3 method were employed. Two mechanisms were considered: a one-step concerted pathway and a stepwise mechanism involving [1,3] and [1,5] hydrogen sigmatropic migrations. The CAM-B3LYP/6-31G(d,p) calculation reasonably agrees with the experimental kinetic parameters. The mechanism appears to be unimolecular in one step concerted through a five-membered cyclic transition state. Isomerization of product cis-bicyclo[4.3.0]nona-2,4,7-triene-9-one yielding 1-indanone is presented and described. Calculation from substrate 7 may explain in a similar way the mechanism of decomposition of compounds 1-6. The present work may well promote to the possibility of carrying out experimental research works on the thermal decarbonylation kinetics in a liquid solution and in the gas phase of β,γ-unsaturated aliphatic ketones.