Hydrogen (H-2) is the most important chain-transfer agent to control the molecular weight (MW) of polymer in all industrial processes using Ziegler-Natta catalysts. However, the specific mechanism of chain-transfer reaction by H-2 is Still open for discussion. Stopped-flow techniques were utilized to investigate the chain-transfer mechanism by H-2 on TiCl4/EB/MgCl2 catalyst pretreated by triethylaluminum (TEA) cocatalyst within an ultrashort time (0-1.1 s). First, the results obtained in the absence of H-2 demonstrated the first experimental evidence of plausible guard effect on the active sites by coordinating growing polymer chain in the initial stage of propylene polymerization. A gradual decrease of active sites concentration, increase of chain propagation rate constant and isospecificity of active sites were observed even after an ultrashort period pretreatment due to the preferential deactivation of aspecific sites, which was found not to be affected by the presence of H-2. On the other hand, H-2 could not decrease the MW of polymer except using catalyst with pretreatment. Moreover, the MW is gradually decreased with increase of pretreatment time and the extent of MW decrease was enhanced with the increase of pretreatment time suggesting that deactivation of polymerization sites could be associated with the formation of H-2 dissociation sites for chain transfer. This was subsequently confirmed by H-2-D-2 exchange reaction combined with simultaneous pretreatment of the catalyst with TEA, which indicates a plausible transformation of polymerization sites into H-2 dissociation sites for chain transfer on Ziegler-Natta catalysts induced by interaction with Al-alkyl cocatalyst.