Dynamics of colloidal flocculation induced by cationic polyelectrolyte was studied in order to evaluate the effective layer thickness of the adsorbing polyelectrolyte under the condition of sufficiently high ionic strength.In this condition, polyelectrolyte chains are expected to be sufficiently flexible taking shrinked conformation of random coils.The size of polyelectrolyte in the solution, ap, was evaluated by the viscometry and by dynamic light scattering (DLS).Drimethylamino ethylmethacrylates with nominal molecular weights of 4.9 million and 2.0 million were tested as flocculants.In contrast to cogulation with a simple salt, the rate of flocculation just alter the onset of mixing with polyelectrolytes was remarkably enhanced then decreased or stopped abruptly after certain period.The protruding length of polyelectrolyte obtained from the enhancement of the rate of flocculation in the initial stage, δHe, was found to be several times longer than the size of polyelectrolyte in bulk solution measured by viscometry and DLS.