Dynamics of poly(alpha -methylstyrene) of high molecular weight M-w = 6.85 x 10(6) in solution of good solvent was investigated at 25 degreesC by dynamic light scattering under Couette flow. The applied shear rate was ranging from 0 to 4.5 s(-1). The present purpose was to inspect the theoretical way of descriptions of flexible-linear-chain dynamics in dilute solution. At the shear rate below 2.8 s(-1), two modes of motions coexisted, i.e., the diffusion motion of the single chain and the intramolecular modes of motions. The decay rate for the diffusion mode increased with increasing the shear rate but that for the internal modes was nearly constant. Whereas, above 2.8-4.5 s(-1), the internal modes were suppressed exclusively and only the center-of-mass translational diffusion of the chain was detected. The observed diffusion was found to be the Brownian motion conducted by a quasi-ideal chain, while the internal motions were suffering the Zimm-type of nondraining hydrodynamic interactions. Moreover, the universal ratio Omega /D(0)q(2) under Couette flow was examined closely as a function of qR(G) because this is a critical measure to inspect how the polymer chain behaves dynamically and how the dynamics can be affected by the hydrodynamic interactions in solution. Here q and R-G are the scattering vector and the radius of gyration of the chain, respectively. The obtained result showed that the ratio was located on the theoretical curve for flexible linear chains in Theta state, where the theory was performed under the microscopic descriptions on both chain segments and solvents with full hydrodynamic interactions. This fact confirms strictly that the coupled kinetic equations for chain segments and solvent in the same dynamic level are indispensable to describe rigorously the dynamics of flexible polymers in dilute solution.