Hysteresis phenomenon in scramjet has been observed in combustion experiments. Its mechanism was seldom investigated. Present works aim to specify the conditions when hysteresis phenomenon occurs in cold flow experiment. Experiments were conducted in a direct-connect isolator part with a wedge in Mach 3 flow. Two different condition in the exit of isolator were considered. The experimental results show that the hysteresis phenomenon appears under the disturbance of wedge. We consider that the disturbances which induce adverse pressure gradient play a dominant role on the occurrence of flow hysteresis. 漏 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.