Thermal stability and the primary initiation mechanism of exo-tetrahydrodicyclopentadiene (exo-THDCP, C10H16) were investigated in a batch-type reactor. The catalytic role of the stainless steel inside the reactor was eliminated by inserting a quartz flask. exo-THDCP decomposed at temperatures over 623 K and 1-cyclopentylcyclopentene (1-CPCP, C10H16) and 4-methyl-2,3,4,5,6,7-hexahydro-1H-indene (4-MHI, C10H16) were the primary decomposition products of exo-THDCP. C-10 hydrocarbons were determined to be the major products. The amount of C-5-C-7 hydrocarbons, such as cyclopentene, benzene, and toluene, were relatively small. We performed the molecular modeling (MM) on some of the compounds, including 1-CPCP and 4-MHI produced from exo-THDCP to evaluate the activation energy and molecular structure of the intermediates. The experimental and MM results showed that 1-CPCP and 4-MHI were independently formed from exo-THDCP. The experimental results closely corresponded with the MM result; the products that were only minimally produced after the reaction had qualitatively higher activation energies than the other products.