Effects of chain ends on the dynamics of atactic poly(methyl methacrylate) (PMMA) was studied by the spin-label technique. PMMA's with well-defined molecular weights were synthesized by atom transfer radical polymerization and selectively labeled at the end or inside sites of the chain with a stable nitroxide radical. The WLF treatment demonstrated that a transition temperature, T-5.0mT, at which an extreme separation width was 5.0 mT, reflected the glass transition observed at a frequency of electron spin resonance (ESR). The T-5.0mT of the PMMA labeled at the chain end (T-5.0mT,T-e) was ca. 8 K lower than that of the PMMA labeled at the inside sites (T-5.0mT,T-i). From the comparison of some polymers, the difference between the T-5.0mT,T-i and T-5.0mT,T-e of a rigid polymer was found to be larger than that of a flexible polymer. The T-5.0mT,T-i and T-5.0mT,T-e decreased with a decrease in the molecular weight as well as a glass transition temperature determined by calorimetric measurements. This result supports that the T-5.0mT reflects the glass transition of the PMMA and suggests that the cooperative motion with neighboring segments is necessary for not only inside segments but also chain ends to undergo a rotational relaxation. The high mobility around chain ends brought a shorter correlation time (tau(c)) at the region around chain ends. Moreover, a model on the basis of an arrangement of segments suggested that an encounter of chain ends was unable to be ignored in the case of low molecular weights, and the encounter of chain ends locally induced a much shorter tau(c) around them.