An analytical separation of submicron colloidal polystyrene latex particles of different sizes has been carried out by capillary hydrodynamic fractionation, We examined various factors which affect separation of the colloidal particles such as the ratio of particle to tube diameter, the average eluant velocity, the ionic strength, and the surfactant concentration, In particular, the electrokinetic interaction between the capillary wall and the particle was studied intensively by measuring the zeta-potentials of the capillary wall and the colloidal particles using capillary electrophoresis, The results showed that the increase in the eluant ionic strength, especially of positive ions, reduced the velocity enhancement factor by the screening effect of positive ions, In addition, we confirmed that sodium dodecyl sulphate molecules, which dissociate in aqueous medium into positive sodium ions and hydrocarbon chains with negative charges, modify the electrokinetic properties of the particles in different way from the case for the capillary wall. For the particles, adsorption of the hydrocarbon chains with negative charges changes the zeta-potential. On the other hand, the screening effect by the dissociated sodium ions is dominant on the electrokinetic properties of the capillary wall.