cis-Polyisoprene (PI) has the type A dipole parallel along the chain backbone so that the end-to-end fluctuation of PI chains results in slow dielectric relaxation. Comparison of dielectric and viscoelastic data of PI has revealed several interesting features related to the entanglement dynamics, for example, success and failure of the full dynamic tube dilation (DTD) picture for monodisperse linear and star PI, respectively [see a review: Watanabe, H. Polym. J. 2009, 41, 929, for example]. For monodisperse linear PI, recent modeling [Glomann et al. Macromolecules 2011, 44, 7430] and single-chain slip-link simulation [Pilyugina et al. Macromolecules 2012, 45, 5728] suggest that the constraint release (CR) mechanism has negligible influence on the dielectric relaxation time τε in the entangled regime, which appears to disagree with the previous data. Thus, we revisited the classical problem: CR contribution to the dielectric relaxation of PI. Specifically, we made dielectric and viscoelastic measurements for PI/PI blends in a wide range of the molecular weights of long and short components, M 2 = 1.1M and M 1 = 21K–179K, and with a small volume fraction of the short component, υ1 = 0.1 and/or 0.2, to examine the CR contribution in the experimentally clearest way. It turned out that τε of the short component was longer in the blends than in respective monodisperse bulk even for M 1 = 179K. Furthermore, the viscoelastic and dielectric data of the short components (M 1 ≤ 43K) in the blend exhibited identical mode distribution and relaxation time, which confirmed that the CR mechanism was fully suppressed for these components in the blends. These results demonstrate that the CR mechanism does contribute/accelerate the dielectric relaxation in monodisperse bulk PI systems even in the highly entangled regime (M 1/M e = 36 for M 1 = 179K). This CR-induced acceleration was found to be consistent with the empirical equations for the terminal relaxation time and CR time of monodisperse PI available in the literature, as noted from a simple DTD analysis of the terminal relaxation process (reptation along a partially dilated tube that wriggles in a fully dilated tube).