Liquid Lump velocities were experimentally determined for horizontal gas-liquid two-phase flow under a wide range of pressures from near the atmospheric pressure to a highly elevated pressure about 20MPa. The liquid lumps dealt here were liquid slug, huge wave, disturbance wave and ephemeral large wave. Velocities were determined using time-series signals of cross-sectional mean liquid holdup which were electrically detected at two axially separated locations of test tube. It is demonstrated how the mean values of liquid lump velocity and its standard deviation depend on the gas and liquid flow rates and pressure. It is obtained that the liquid lump velocity and its standard deviation are obviously affected by the pressure and are related flow patterns. The comparison of liquid lump velocities between vertical upward flow and horizontal flow are also discussed. The comparison has been also made between liquid lump velocities of vertical upward flow and horizontal flow.