The ESR spectral change in spin-labeled poly(methyl methacrylate) (PMMA) was investigated in the solid phase at various temperatures. There appeared a shoulder on the spectrum from 138°C to 188°C and it is concluded that this shoulder originates from the existence of the two kinds of motion in this temperature region. It was also found by GPC measurements that the molecular-weight distribution has no relationship to the appearance of this shoulder. The data of the second moments of the PMMA of the 1H NMRspectra indicate that the two kinds of motion were the main chain motions of PMMA. Our results suggest that there is a difference in the aggregation state of PMMA molecules.