Abstract: Gas transport mechanism is considerably complex in micro fractures of shale and tight gas reservoirs, which is caused by a fracture with nano-micro scales aperture. A unified model for gas transport in micro fractures of shale and tight gas reservoirs is proposed by coupling slip flow and Knudsen di usion, where, the ratios of the intermolecular collision frequency and the molecule-wall collision frequency to the total collision frequency are the weight coe cients of slip flow and Knudsen di usion, respectively. The present model considers the e ects of micro fracture shape and dimension on gas transport, and is validated with molecular simulation data published. The results show that: (1) the present model can describe all known gas transport mechanisms in micro fractures, including continuous flow, slip flow and transitional di usion; (2) the present model can reasonably describe the process of gradual mass transform for di erent gas transport mechanisms in micro fractures during di erent development stages of gas reservoirs; and (3) The micro fracture shape and dimension significantly impact gas transport, at the same micro fracture aperture, the greater the micro fracture width, the stronger the gas transport capacity, which is more obvious in the case with higher pressure and greater micro fracture aperture.