An experimental investigation was made of the mixing of liquids in a tank where the liquid jet was injected through a nozzle. The mixing time was defined as the time required to reduce the concentration variation within 1% of the mixed mean value and was measured by an impulse response. The circulation time was also obtained from the response curve. It was found that in the circulation flow regime of mixing (Re>3 ×104) there exists an optimum nozzle depth for rapid mixing. It ranges from the liquid surface level to three-quarters of the liquid depth when the liquid depth is equal to the tank diameter, and is the mid-depth of the liquid when the liquid depth is smaller than the tank diameter. When the nozzle height is within one-fourth of the tank diameter, it is efficient for rapid mixing to tilt the nozzle upwards enough to prevent the formation of a wall jet, which induces circulations of small variance of circulation time.