Abstract: The wake of a circular disk in a water channel during starting was observed by hydrogen bubble method with two different disks while the accelerations of the free stream were approximately constant during the starting stage.In the starting stage the vortex sheet separated from the disk edge rolled up into a helical vortex ring named starting vortex as shown in Fig. （a ） to （ f ）. Both its diameter dv and abscissa xv increased linearly （Figs 5 and 6 ）. Induced by the starting vortex, the flow in the center of the wake reversed （Fig. 8 ）. But the abscissa of the rear stagnation point x, also increased linearly （Fig. 7 ）. The back velocity peaked at the point the starting vortex was passing through （Fig. 9 ） and attained its extreme while the intensity of the starting vortex reached its maximum at the end of the, starting stage. This extreme（1.3U ）far exceeded the maximum hack flow speed of the, wake in steady flow. Therefore, it may be of great value to the drag and virtual mass of the disk.In the transition stage the vortex sheet no longer rolled up into the starting vortex and the starting vortex became, unsymmetric and broke up as shown in Fig. 3 （g ） to （ i）. In the, meantime the wake was able to swing rather far from its centerline （Fig. 12） and the flow appeared very chaotic and violent.After the transition stage, the flow became quasi-steady and the flow inside the, wake was turbulent. There only occurred small vortices around the wake and they were no longer converged into a large vortex like the starting vortex （Fig. 11）. The profile of the wake approached to symmetric.