The mean relaxation times, T-m’s, of four different polymers measured by the fluorescence depolarization method were compared at their Theta temperatures. These polymers, i.e., cis-polyisoprene (cis-PI), polystyrene (PS), poly(alpha-methylstyrene) (P alpha MS), and syndiotactic poly(methyl methacrylate) (s-PMMA), were labeled with anthracene in the middle of the main chain by living anionic polymerization. The Theta temperature is a standard point without an excluded volume effect, and the Theta temperature of these four polymers are almost the same, 34-35 degrees C. Therefore, the comparison of the T-m’s at the Theta temperature enables us to clarify the relationship between the chain local motion and the chemical structure. The order of the reduced relaxation times, T-m/eta, was T-m/eta(cis-PI) < T-m/eta(PS) < T-m/eta(P alpha MS) T-m/eta(s-PMMA), and the order of activation energies, E*, was E*(cis-PI) < E*(PS) < E*(P alpha MS) < E*(s-PMMA). From a molecular standpoint, the relaxation times depend on both the substituents attached to the main chain and the stereoregularity of the polymer chains. In consideration of the stereoregularity of the polymer chains, the relaxation time of atactic P alpha MS is estimated to be not much different from the relaxation time of atactic PMMA.