Abstract: Experimental and theoretical investigations of water flow resistance in flat tube were carried out. The approximate solution of the friction factor was derived theoretically in the laminar region. And the friction factor was also correlated with the experimental data in the turbulent region. The results show that the friction factor of flat tube varies similarly with that of the circular tube, whereas the former is higher than the latter. And also, Reynolds number of transition area from laminar to turbulent region is higher than circular tube. Both the theoretical expression and the empirical correlation are accurate very much, and the deviations are less than 4%.