This research was about anti-polymerization both in the purification process of high purity cyclopentadiene from C5 fraction and in synthesizing norbornene via Diels-Alder reaction. During the purification process of HPCPD, high reaction temperature reduced polymerization reaction as a side reaction better than low temperature. The TISP among the inhibitors got the higher rate of CPD conversion, and over 100 ppm of the concentration of inhibitor was suitable for stable CPD production. In addition, the synthesis of norbornene using inhibitors reduced the side reaction, but the selectivity of endo and exo norbornene form was not affected by the use of inhibitors.