A refrigerant tube (T1) for use in heat exchangers comprises a flat aluminum tube (5) having parallel refrigerant passages (4) in its interior and comprising flat upper and lower walls (1,2) and a plurality of reinforcing walls (3) connected between the upper and lower walls (1,2), extending longitudinally of the tube (5) and spaced apart from one another by a predetermined distance. The reinforcing walls (3) are each formed with communication holes (6) for causing the parallel refrigerant passages (4) to communicate with one anther therethrough. The flat aluminum tube (5) is prepared from upper and lower two aluminum sheets by bending opposite side edges of the lower aluminum sheet to a raised form and joining the bent edges to the respective side edges of the upper aluminum sheet which is flat so as to form a hollow portion. The reinforcing walls (3) are formed by joining to the inner surface of the upper wall ridges projecting inward from the lower wall. The communication holes (4) are formed by cutouts formed in the edges of the ridges at a predetermined spacing and having their openings closed with the upper wall.