A novel method based on a reaction between the growing polymer chain and cinnamoyl chloride (CC) was developed to determine the number of active centers ($C_p$) in olefin polymerization with supported Ziegler-Natta catalysts. The reaction of titanium-polymeryl bonds in the active centers of $MgCl_2$/ID/$TiCl_4-AlR_3$ type catalysts with CC was very fast at $20{sim}80^{circ}C$. Cinnamoyl labeled polymer chains were formed, which were stable in the reaction system for at least 10 min when the CC/Al molar ratio was > 1. The number of active centers was determined by measuring the number of carbonyl groups in the polymer with the UV-vis method. The $C_p$ values of the 1-hexene polymerization systems with the addition of hydrogen were also measured.