The average boundary layer thickness at the side wall of an agitated vessel without baffles, y(D), was estimated from the correlation of the friction factor with the help of the measured universal expression of tangential velocity distribution at a vessel wall. The average boundary layer thickness was independent of the Reynolds number in the turbulent boundary layer range of Re-G > 5000, but it increased with decreasing Reynolds number being inversely proportional to the 1/2 power of a generalized Reynolds number in the laminar boundary layer range of Re-G < 5000. At the turbulent boundary layer of Re-G > 5000, the ratio of the average boundary layer thickness to the vessel radius y(D)/(D/2) was in the range of 0.013 to 0.018 for the ratio of impeller to vessel diameters d/D = 0.61 to 0.4. The correlation line of the dimensionless tangential velocity at the boundary layer edge u(D)(++) ( u(D)(+)(ss D/md)) vs. the dimensionless boundary layer thickness y(D)(++) ( y(D)(+)(ss D/md)) coincided with the universal velocity distribution law for a flat plate for the impellers having a larger impeller similarity parameter X over 0.15, but the former shifted upward from the latter for the impellers having a smaller impeller similarity parameter X below 0.15, although the gradient (du(D)(++)/d ln y(D)(++)) Was the same as that for the universal velocity distribution law.