A layered heat exchanger for use as a motor vehicle air conditioner evaporator comprises pairs of generally rectangular adjacent plates, which are joined together in layers with the corresponding recesses of the plates in each pair opposed to each other to thereby form juxtaposed flat tubes each having a U-shaped fluid channel, and front and rear headers in communication respectively with opposite ends of each flat tube. The turn portion of U-shaped fluid channel of the flat tube has a fluid mixing portion comprising many small projections, and a rectifying portion comprising parallel long projections along a flow of fluid. The channel turn portion rectifies the flow of fluid and mixes the fluid at the same time, permitting the fluid to flow through the turn portion smoothly to result in a diminished fluid pressure loss, an improved heat transfer coefficient and improved performance.