Abstract: Axial mixing coefficients in gas phase under nitrogen-water two-phase flow conditions at high pressure were measured in a packed column (0.15m diameter,8.4m high, with a 2.1m packed section) by the technique of pulse injection of hydrogen. The packing was 350Y Mellapak, manufactured by the Tianjin UnivTech Co. Ltd.. Nitrogen and water flowed countercurrently through the column at gas rates from 0.03 to 0.15m·s^-1 ,and liquid rates from 17.0 to 75.0m³·m^-2 ·h^-1 . The operation pressure varied from 0.10 to 1.70MPa.The hydrogen concentration in gas phase was detected continuously by means of thermal conductivity. The one dimensional diffusion model was used for smoothing the experimental response curves with 7 per cent average errors. The experimental data showed that the axial mixing coefficient in gas increased with the increase of the interstitial velocity and the operation pressure. This trend may be due to the increase of the density and the viscosity of the nitrogen under high pressure. Practically no effect of the water loading on the axial mixing coefficient was observed.