Tiny amounts of 1,5-hexadiene (1,5-HD) are introduced into propylene bulk polymerization using Ziegler-Natta catalyst. The thermal stability can be enhanced by possible backbone cyclization which has inconspicuous change to the backbone structures of polypropylene. MgCl2 /TiCl4 catalyst with 9, 9-bis(methoxymethyl) fluorine (BMMF) as internal electron donor is used for bulk polymerization and concentrations of 1, 5-HD are controlled well below 0. 1 mol/L. Nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), dynamic shear rheology and scanning electron microscopy (SEM) are used to characterize the structures and morphology of the polymer. The results indicate that, albeit the catalytic activity and polymer molecular weight exhibit certain degrees of decrease due to 1, 5-HD, the major crystallization properties of polypropylene including melting temperature (T.),fusion enthalpy during melting (AH,) and maximum crystallization temperature (To) remain almost unchanged. Because of the tiny amounts of 1, 5-HD, the structure of 1, 5-HD in PP cannot be detected by neither(1) H-NMR nor(13) C-NMR. The isotacticity of PP has little change by(13)C-NMR. The structure of polymer is linear which is characterized by GPC and dynamic shear rheology analysis and no branched or crosslinking structure is formed in polymerization. The insertion of 1, 5-HD has little influence to the chain structure of polypropylene macromolecules. Thermogravimetric analysis (TGA) results indicate that the thermal stability of polypropylene is much improved by polymerizing with 1,5-HD. The onset degradation temperatures (T-onset,) of polypropylenes obtained with the presence of 1, 5-HD are increased by as much as 20 K, from 460. 6 degrees C to 482. 3 degrees C under nitrogen atmosphere and 5% and 10% weight loss temperature increased about 30 degree. It indicates that tiny amounts of 1,5-HD is indeed inserted into the macromolecule chain as the cyclic structure which enhances the rigidity and improves the thermal stability of the polymer. Moreover, polymer particles in reactor are granule-like, which meets the demand of industrial polymerization process. Overall, this research provides new thinking to improving thermal stability of polypropylene resins.