This paper describes the experimental results on boiling heat transfer and frictional pressure drop of high pressure steam-water two-phase flow in internally ribbed tubes used in once-through boilers of large thermal power plants. The test tubes are 4 thread internally ribbed tubes with diameter of 28×6mm and 22×5mm. The experimental parameters are: pressure p=13∼27Mpa; mass velocity G=400∼1800 kg/m2.s; and steam quality x=0∼l. The boiling heat transfer is measured in heated vertical upward tubes and the frictional pressure drop is measured in adiabatic horizontal tubes. Based on the tests, the wall temperature profiles of internally ribbed tubes are obtained; the critical heat flux, critical steam quality and critical mass velocity at the beginning of boiling heat transfer deterioration are given; the correlation of boiling heat transfer coefficient, post dryout heat transfer coefficient and two-phase frictional pressure drop are established. The results may be recommended to be used for the design of large steam boiler.