The dielectric relaxation of poly(phenyl methacrylate) and two poly(methoxy-substituted phenyl methacrylate)s was studied over the frequency range from 30 Hz to 3 × 105Hz and the temperature range from −100°C to 170°C. Each sample shows three relaxation processes, termed α, β, and γ, within the temperature ranges 90° to 130°C, 20° to 90°C and −60° to −20°C, respectively. The α and β processes were attributed to the micro-Brownian motion of the main chain and the entire side chain, respectively. The γ process was suggested to be associated with the motion of methoxy phenyl groups at the end of the side chain.