An experimental and theoretical study has been made on turbulent flows in the entrance region of a circular tube. Direct measurement of shear stress at the tube wall was made by an electrochemical method, and velocity profiles at various cross sections were measured by a total pressure tube. From plotting these results in universal coordinates, it is found that there exists a similarity in the velocity distributions in the turbulent boundary layer in the entrance region, and that they are represented by the power law equation. The coefficient and exponent of this equation are functions of the Reynolds number based on inlet velocity only. Based on the assumption of this similarity, the momentum equation for a circular tube is solved. Analytical solution agrees fairly well with the experimental results on turbulent boundary layer developments.