Molecular mobility of polyimide brushes, grafed polymers with long polytretbuthylmethacrylate side chains, was investigated by dielectric method. In the glassy state two local relaxation processes, β and β1, whose molecular mobility depends weakly on side chain lengths, were found. In the viscous elastic state the α cooperative relaxation process caused by segmental motion of side chains was observed. For the α process, the molecular mobility decreases with the length of polytretbuthylmethacrylate side chains.