<p>The fluid flow characteristics in fractures exhibit size dependence. The effects of fracture size and its roughness condition on the hydraulic conductivity of rock fractures was investigated in the study through numerical simulation and laboratory experiments. Based on the fracture data of field scanning, a fracture model for fluid flow simulation and experimentation was established. The permeability characteristics for different-sized fracture were obtained through numerical simulation based on the fracture model. The permeability characteristics parameters varied with the fracture size and significant variations mainly occurred for small-size fracture. The permeability tended to stabilize for large-size fractures, while the non-Darcy coefficient gradually increased. By comparing the hydraulic properties and fracture surface roughness, the permeability and non-Darcy coefficient were highly correlated with surface roughness. A model for size effect on flow behavior in rock fracture considering modification coefficients <i>A</i> and <i>B</i> in the Forchheimer equation was proposed. The coefficient A was negatively correlated with roughness and the expression was established based on the relation between permeability and roughness. The coefficient B was related to roughness and fracture size and was determined based on relation among non-Darcy coefficient, roughness and fracture size. Finally, laboratory fluid flow experiments with different fracture sizes were performed to verify the correctness of the proposed model. However, the fracture sizes in simulation and experiment was within 1&#xa0;m and larger models need to be established to the effects of fracture size and its roughness condition on the hydraulic conductivity in the future.</p>