Mass spectra of the dammarane triterpenes having open side chain and $C_{20}-C_{25}-epoxy$ side chain were measured. Principal fragment ions were assigned and plausible mechanisms for the formation of the fragment ions were proposed. In general, the triter-penoids of $C_{20}-C_{25}-epoxy$ side chain. produce $h_{1}-species$ fragment ions by the deletion of side chain at $C_{20}-C_{22}$ bond and open side chain triterpenoids produce $h_{2}$ species fragmentions whose mass numbers are higher by two mass unit than those of $h_{1}$ species. The mass number of h species fragment ions will serve as the diagnostic tool for the elucidation of side chain structure of tetracyclic triterpenoids.