Nitro-phenyl maleimide (NPM), is the organic stabilizer for poly (vinyl chloride) (PVC), has been investigated as thermal plasticization for rigid chlorinated poly (vinyl chloride) (CPVC). Dielectric relaxation of CPVC stabilized with 10 wt% of NPM has been studied in temperature and frequency ranges of 300-450 K and 10 kHz-1 MHz, respectively. An analysis of the dielectric constant, epsilon and dielectric loss index, epsilon, was performed assuming a plasticization effect of NPM molecules. The plasticization effect of NPM molecules was confirmed by the behavior of the dielectric modulus M and M spectra. A clear dielectric -relaxation process has been obtained in the studied temperature range. The results showed that NPM reduce the glass transition temperature, T-g, of CPVC by about 20 K. This effect has been assigned to the plasticization effect of NPM. At lower temperatures, dielectric modulus spectra reveal that there is a role of the effect of the electrode polarization in the relaxation process. The behavior AC conductivity, sigma(ac), indicated that the conduction mechanism in all CPVC samples is hopping type conduction. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers