The mechanisms of all the addition reaction paths of 1,3-cyclohexa-diene with propylene have been studied by using ab initio UHF/6-31G* method. All geometries of the stationary points have been optimized. Stepwise and concerted processes are both possible for the reaction. The stepwise processes are most favorable in all reaction paths. The exo and endo products of the reaction can be formed through two mechanisms, i.e. the terminal double-bonded carbon of propylene attacks 1,3- cyclohexa-diene at first step or so do the center double-bonded carbon. The stepwise processes contain four reaction paths involving biradical intermediates. The calculated activation barriers of the rate determining steps of these paths are about 102~114 kJ•mol-1. Based on microcanonical transition state theory the canonical rate constants for the four stepwise paths have been computed by using data calculated at UHF/6-31G* level. The computational results show that the canonical rate constants for the reaction paths of forming endo product are larger than that for exo ones, and that the canonical rate constants for the reaction paths with the terminal double-bonded carbon of propylene attacking 1,3- cyclohexa-diene at first step are larger than that with the center carbon atom. The canonical rate constants of four stepwise paths are close to each other, so they are competing reactions.