Chain dynamics of poly(&agr;-mehylstyrene) of high molecular weight in benzene, a good solvent, in dilute solution was investigated by dynamic light scattering under Couette flow. At the shear gradient above 2.8–4.5 s−1, the internal modes of motions were exclusively suppressed and only the center-of-mass translational diffusion motion of the chain was detected. Whereas, in the intermediate shear region, the decay rate for the internal mode was constant but that for the diffusion mode increased with increasing the shear rate. The obtained universal ratio&OHgr;/D0q2was located close to the theoretical curve predicted for flexible chains under themicroscopicdescription of chain dynamics in the &THgr; state. This quantitative agreement between theory and experiments means that the coupled kinetic equations for chain segments and solvent in the same dynamic level is indispensable for describing rigorously chain dynamics in dilute solution. ©2000 American Institute of Physics.