A new definition of mixing time was proposed based on the fraction of well-mixed volume. The mixing process in the stirred tank with a single Rushton turbine was examined by solving the transport equation of the tracer based on the numerical simulation of the whole flow field. It was shown that the value of mixing time was dependent on the speed and position of impeller. Mixing time decreased with the increase of impeller speed at the same volume concentration. When the impeller was in the center of the tank

the mixing time was the shortest. The draft tube with proper dimension and position could effectively decrease mixing time.