Muon spin relaxation (SR) measurements were performed to investigate the dynamics of polybutadiene (PB) reinforced by zinc diacrylate (ZDA). The spin relaxation rate was found to be almost independent of temperature below the glass transition temperature T-g, while it increased monotonically with temperature above T-g. In addition, the spin relaxation rate for PB including 14 vol% ZDA, which has a higher elastic modulus, was slower than that of PB without ZDA. Thus, SR can clearly distinguish the dynamics of PB in these two samples, with the spin relaxations mainly caused by the so-called a-process (above T-g) and fast beta-process (below T-g). To examine this scenario in detail, we performed SR experiments as a function of longitudinal magnetic field (LF). The motional relaxation rate for PB was independent of temperature below T-g, confirming that the spin relaxation is mainly induced by the fast I3-process. The a process above T-g was also confirmed based on the Vogel Fulcher-type temperature dependence, and the motional relaxation rate in the a-process was decreased by ZDA aggregates. These findings clearly show that SR measurements provide useful information on the dynamics in complex polymer systems. (C) 2016 Elsevier Ltd. All rights reserved.