The competitive four-channel thermal decomposition reaction of 1,2-dibromopropane was studied in the fall-off region at 605.5K. Four monobromopropenes were obtained by hydrogen bromide elimination process in the reaction. The product ratios showed a pressure dependence and the relative amount of various isomers at 1 torr were 73.8\% of 3-bromopropene, 10.3\% of 2-bromopropene, 10.1\% of cis-1-bromopropene, 5.8\% of trans-1-bromopropene. The RRKM formalism was used to compare with the experimental results quantitatively. The pressure dependence on product ratios was well described by the calculation.