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We conducted dielectric and viscoelastic relaxation measurements for polystyrene (PS)/4-pentyl-4′-cyanobiphenyl (5CB) and PS/4-pentyl-4′′-cyanoterphenyl (5CT) mixtures in the miscible state with the weight fractions of 5CB or 5CT from 0.04 to 0.27 to examine the relationship between two-component dynamics in the mixture. Dielectric relaxation measurements mainly probed the dynamics of low-mass molecule (LM), 5CB or 5CT, because of the much larger dipole moment of LMs compared to that of PS. PS/5CB mixtures exhibited bimodal dielectric relaxation, while PS/5CT showed unimodal relaxation. The observed two relaxation modes (slow and fast modes) in PS/5CB were attributed to the full rotational motion and partial rotational motion of 5CB, respectively. The latter motion is allowed in the restricted space surrounded by less mobile PS matrix. In the case of 5CT, the fast mode was suppressed due to the larger size of 5CT molecule, and only the slow mode (rotational mode) appeared. The relaxation times of slow mode for 5CB and 5CT had approximately the same (non-Arrhenius-type) temperature dependence with the viscoelastic relaxation time for the terminal flow zone of PS at high temperatures (at least 20–30 K higher than the glass transition temperature). This result suggested that the slow modes of 5CB and 5CT are cooperative with a global motion of PS-chain. In contrast, relaxation time for the fast mode observed in PS/5CB had weaker (Arrhenius-type) temperature dependence than the slow mode, which indicated that the fast mode of 5CB is decoupled with the PS chain motion. [ABSTRACT FROM AUTHOR]