Abstract: The effective permeability of rock fractures is a fundamental parameter for describing unsaturated flow and multi-phase flow in fractured media, and the fracture aperture is an important factor affecting this parameter. In this paper, to investigate the effect of aperture on the flow structures of water-oil multiphase flow and on the effective permeability, we develope a visualization experimental system, and perform multiphase flow experiments in fracture models replicated from real rock fractures with three different apertures. Visualization experimental results show that the flow of non-wetting phase in the fracture can be categorized as unstable bubble flow at the low flow-ratio conditions and stable channel flow for high flow-ratios. As fracture aperture increases, the flow channel of non-wetting phase becomes less branching and wider, and the effective permeabilities of the two phases both increase, during which the flow structures become stable. The visualization results also reveal the competing mechanism of fluid-fluid alternately occupying the fracture space in the slug flow structure. When the non-wetting phase fluid channel changes from continuous to discontinuous, the pressure difference between the inlet and the outlet of the fracture increases significantly; conversely, when the channel changes from discontinuous to continuous, the pressure difference decreases significantly. Finally, based on the fractal theory and the statistical model for permeability, the effective permeability model proposed for multiphase flow in rock fractures with variable apertures, and the correctness and reliability of the model is evaluated by the measured effective permeability data.