We focused on molecular motion around side chain ends of multibranched polystyrene (Mb-PSt) by a spin label method of electron spin resonance (ESR). The Mb-PSt was synthesized via living anionic polymerization with macroinitiator technique and the chain ends were modified with TEMPO. Effect of the side chain length on the mobility of the chain ends was studied by the ESR method. The results indicate that shorter side chains have higher mobility at the chain end.