This paper describes an experimental and numerical simulation study of the unsteady flow of the starting process of a two-dimensional nozzle. The analysis focuses on changes in the flow-separation point, in addition to the overall flow structure. The position of the separation point in the nozzle-starting process is simulated using a turbulent model and an appropriate arrangement of the computational grid. It was found that the flow structure near the nozzle throat wall is important in the nozzle-starting process. The boundary layer including the vortical structure in this area characterizes the flow structure of the downstream side. Moreover, the aspect ratio of the computational grid should be 1:1 near the nozzle throat wall in order to reproduce the results of the nozzle-starting experiment. Furthermore, a simplified RANS turbulence model cannot reproduce the separation point in the nozzle-starting process.