The liquid velocity in the riser of the draft-tube airlift reactor was investigated. The liquid circulation was intensified by the mechanical mixing. The propeller was placed under the gas sparger. The influence of the gas flow, rate, the rotational speed of the impeller and the liquid viscosity were tested. The tracer method was used to determine the liquid velocity. The delay time between the response signals of the fixed and the experimentally translocated sensors was measured. An experimental equation for the calculation of the riser liquid velocity was proposed.