The variation of polymer chain dimension with concentration in both dilute and extremely dilute solutions was monitored by measuring the diffusivity of narrowly distributed polystyrene standards in toluene at room temperature through a carefully designed dynamic light scattering experiment. As polystyrene concentration decreases, the measured translational diffusion coefficient levels off after a certain concentration, i.e., in terms of experimental results, there exists a "critical" concentration for polystyrene coil shrinking in dilute toluene solution. The existence of this critical concentration has been predicted by introducing the concept of screening length into the statistical thermodynamic calculation. In this paper, we report, for the first time, its laser light-scattering evidence.