The addition of foreign substances is well-known to be able to improve the crystallization of isotactic polypropylene (iPP) following a nucleation-promoting mechanism. In the present work, however, we found that zinc salt of 1, 4, 5, 6, 7, 7-hexachlorobicyclo [2.2.1] hept-5-ene-2, 3-dicarboxylic acid (HCHD-Zn) unexpectedly decreased the crystallization temperature (T-cp) and crystallization rate of iPP, generating remarkable antinucleation effects. The results of differential scanning calorimetry (DSC) and polarized optical microscopy (POM) showed T-cp of iPP decreased more than 4 degrees C and nucleation densities of iPP decreased nearly 70% upon the addition of the "anti-nucleating agent" HCHD-Zn. Isothermal crystallization kinetics indicated HCHD-Zn lowered the crystallization kinetic constant k of iPP but had similar Avrami exponent with neat iPP, which revealed HCHD-Zn hindered the crystallization of iPP but did not change the type of nucleation and growth geometries of iPP. We proposed a mechanism of antinucleation of iPP upon the addition of HCHD-Zn from a structural perspective. Hydrogens on the tertiary carbons in the melt of iPP may react with the chlorine in the HCHD-Zn particles dispersed within. The removal of hydrochloride left a 3D network of iPP chains cross-linked by HCHD-Zn particles. Therefore, the restricted movement of polymer chains inhibited the nucleation of iPP.