This paper reports on the performance of a sub-THz liquid sensor tool coupled to a microfluidic platform. Non-invasive, non-destructive and on-line measurements are demonstrated for the determination of ethanol concentration and for controlling bymicromixer device. We have developed a label-free chemical sensing methodology coupling a sub-terahertz sensor technology in microfluidic devices fabricated on glass and polydimethylsiloxane. The concept of sensor in micromixer applications operating at 60 GHz for known ethanol concentration ranging 0% to 100%, with a corresponding dynamic range of 2.79dB. The sensor allowed establishing the correlation between the mass flow rate and the ethanol concentration in the microfluidic system, where two-stream lines form the mixture in laminar flows, with Reynolds number < 13.54 and Peclet number > 25700. We demonstrated on-line sensing and linear control of ethanol concentration on demand with a small variation of 0.32% (v/v) between measured and required ethanol concentration.