Abstract: The friction behaviors for the completely laminar and turbulent boundary-layer flow of incompressible fluid in the bellmouth entrance region of a smooth circular tube had been treated respectively [1,13]. A simplified model (Fig.2) is suggested to determine the entrance effect on the flow friction for the complicated laminar-to-turbulent boundary-layer flow of incompressible fluid through circular tube. The analytical relations obtained previously[1,13], eqs. (1)-(7), are used as the basis for calculation. With the apparent crihcal Reynolds number Rex1 taken as 5 x 105, [4]The predicted pressure drops are in good agreent with the experimental data reported[6], (Fig.3).