In milli/microchannel, fluid flows are essentially laminar and, as a consequence, heat transfer is limited. An important application of millichannels is mixing of several flows. In this article we present mixing of two fluid flows at two different temperatures with the objective of strongly decreasing the time needed to obtain a mixture at a homogeneous temperature. To increase heat and mass transfer, we propose an active enhancement technique by generating secondary flows by using ultrasonic waves. A mixer has been realized consisting of a millichannel fed by two fluids coupled to a piezoelectric transducer with a vibrating frequency (37.7 kHz) adapted to the channel width. A test bench has been realized to measure mixing properties of the mixer with and without ultrasound for specific equivalent Reynolds number (Reeq⩽23). At this frequency, vortices were formed to break the flow laminarity by generating a velocity field with significant transverse components. The observed patterns revealed vortices parallel with the transducer surface corresponding to modal Rayleigh-like streaming. To characterize the mixing performance a thermal mixing criterion (CM) based on input/output temperatures was defined which showed a significant increase in the presence of Rayleigh-like streaming patterns. © 2019 Elsevier Ltd