Fractional pressure drops of single-phase water and steam-water two-phase flows were studied in a pressure range of 3.0-3.5 Mpa. Two helical coiled tubes were employed as test sections and their four different helix axial inclinations were examined. It is found that helix axial angles have little influence on the single-phase frictional pressure drop, while variation of the steam-water two-phase flow frictional pressure drop is enlarged to 70%. For single-phase flow, some previous correlations were quite accurate in predicting the frictional pressure drop for lower Reynolds number conditions, and a modified correlation was obtained from an enlarged Reynolds number range of the present test. For boiling two-phase flow, great deviation was found among the published few empirical equations and their capability of practical utilization is rather poor on account of their complication in structure. A new easily stipulated correlation is deducted from our present data.