Velocity similarity laws based on a four‐layer mean velocity profile model are deduced for turbulent boundary layers with dilute polymer solutions by means of pipe‐flow experiments. Measured drag reduction is found to have three domains: Undersaturated, optimal, and oversaturated. The drag reduction does not increase with increasing concentration in the oversaturated domain where a strong interactive layer dominates the entire logarthmic region of the boundary layer. Drag reduction increases with increasing concentration in the undersaturated domain where the four‐layer profile exists in the boundary layer. The boundary between the two domains gives optimal drag reduction, and is determined by the polymer type and concentration, and by a Reynolds number based on shear velocity and boundary‐layer thickness. Pipe‐flow experiments were made to study the drag‐reduction characteristics in the undersaturated domain. The effects of solvent temperature, pipe diameter, polymer type and concentration, and wall shear stress on the measured drag reduction were investigated.