A tube for heat exchangers is a flat tube which either comprises a pair of plane walls which are spaced a predetermined distance from one another, the plane walls respectively having lateral ends integrally connected to each other by a U-shaped bent portion, the plane walls further having other lateral ends which abut against and are tightly secured to one another, or alternatively, the flat tube comprises a pair of preformed plates having abutted and soldered portions at both lateral ends. The tube further comprises one or more curved lugs integral with and protruding inwardly from an inner surface of each plane wall, and the curve lugs respectively have innermost tops so that the innermost tops protruding from one plane wall bear against the inner surface of the other plane wall or against the innermost tops of the other curved lugs protruding from said other plane wall. Th tube is thus of an improved pressure resistance despite its minimized height or thickness, and the manufacturing process of the tube permits the production of tubes at a high productivity and lower manufacturing cost.