Abstract: The starting process of the flow in a Mach 2.7 supersonic nozzle is experimentally investigated. The starting shock moves from the throat to the exit of the nozzle when the incoming total pressure is gradually increased. The flow in the nozzle undergoes the process of normal shock at the throat, normal shock at the expansion duct, oblique shock at the expansion duct, Mach reflection at the exit of the nozzle, regular reflection at the exit of the nozzle and the design conditions. The transformation from the Mach reflection to the regular reflection is observed based on the experimental result of schlieren and PIV. The three-dimensional flow structure of the leading shocks of the shock train has been rebuilt by using the schlieren image system and the shear-sensitive liquid crystal technology.