Abstract: Both the additional inertial force as well as the pressure drop induced by it on a two-phase flow system in tubes with vertical, horizontal and tilted layouts had been derived in detail. And correlations were given for calculating the additional force and pressure drop under the three layout conditions. Comparing the fluctuation amplitude of additional inertial force and gravity in flow direction, it was found that the gravity is a critical factor in determining the flow characteristics. The correlation for calculating the additional pressure drop in a vertical pipe were evaluated against the experimental data, showing that the correlation could well predict the variation of additional pressure drop in rolling motion.