A local segmental mobility was determined by electron spin resonance (ESR) spin-label method for a series of polystyrene (PS) with various molecular weights. Each PS specimen was selectively spin-labeled with stable nitroxide radicals at a chain end or inside sites. Molecular motion at the inside of the chain was compared with that at the chain end from the temperature dependence of ESR spectra of the nitroxide radicals. The transition temperature of molecular motion, T 5.0mT, at which the extreme separation width due to 14 N anisotropic hyperfine splitting is 5.0 mT, increased with an increase in molecular weight. The WLF equation confirmed that the T5.0mT correlated with a glass transition temperature, Tg, of PS. The T5.0mT for the spin-labeled PS at the chain end was ca. 5 K lower than that for the spin-labeled PS at the inside sites due to the enrichment of the specific free volume around the chain end. The transition temperature, T5.0mT, for both labeled PS depended on the molecular weight in accordance with the Unberreiter-Kanig equation for a glass transition. The T5.0mT for the spin-labeled PS at the chain end had a strong dependence on the molecular weight as compared with that at the inside sites because the molecular motion of the chain end was accelerated by an encounter of more than two chain ends. From the molecular weight dependence, we determined the short correlation time for segmental motion of the chain end, ca. 40 s, and the segment size undergoing the segmental motion at the T g. The obtained segment size agreed well with the general segment size reported by others, 5-10 monomeric unit size.