This paper presents an experimental investigation of the pressure drop and friction factor of supercritical water in an annular channel. The gap and the full length of the annulus are 4 mm and 1400 mm, respectively. The experimental pressure ranges from 23 to 28 MPa, mass flux ranges from 700 to 1500 kg/(m(2)s), and heat flux on the inner wall ranges from 200 to 1000 kW/m(2). Results showed that the frictional pressure-drop increases significantly with increasing mass flux, particularly when the bulk enthalpy surpasses the pseudo-critical enthalpy. A local hump in the friction factor was observed corresponding to the pseudo-critical enthalpy, which becomes stronger with the decrease of mass flux or pressure. The assessment of correlations demonstrated that constant-property-based correlations fail to predict the friction factor of supercritical water. Hence, an improved correlation was proposed which captures 82.6% of the experimental data within 25% error band.