Steady state immiscible oil/water flow experiments were conducted in horizontal smooth-walled fractures. The effects of fracture aperture, injection rate and injection method on the immiscible flow characteristics were investigated. The experimental data were analysed using a porous medium approach and an equivalent homogeneous single-phase method. The pressure gradients predicted by the latter are in good agreement with those measured in the experiments. Contrary to the well-known Romm＇s relative permeability versus saturation correlation, the experimental data indicated that the sum of the relative permeabilitiesis less than one over a range of saturation, which is attributed to the effect of phase interference on the immiscible flow. The effect of phase of interference is more significant in the case of well mixed oil/water flow than otherwise. The results also indicate that the fracture aperture does not have a significant influence on the relative permeabilities of oil and water flow in smooth-walled fractures.