Abstract: To disclose the mechanism of macro laws of fractured lost circulation and microscopic flow character in fracture, the lost circulation laws through smooth fracture while pressure differential, fracture width and drilling fluid viscosity changing were studied by experimental method, and the effect of undulating rough features of natural fracture surface to lost circulation laws was analyzed through CFD simulation. Then the features of influence factors to lost circulation were discussed. The results indicated that the dominant factors affecting leakoff rate were fracture width and pressure differential, and drilling fluid viscosity was a weak factor. The overall undulating of fracture surface making it contacted locally was the main reason of forming effective flow path, while the dominant factors forming effective flow path for micro fracture was the distribution and shape features of asperities. Natural fracture surfaces contacted partially, resulting in flow resistance increase, flow path being more tortuous, so the leakoff rate decreased. It provided a theoretical basis for lost circulation evaluating, leakoff shutoff material selecting, and lost circulation controlling.