A liquid-cooled heat dissipater is composed of a fin, a substrate, and a plurality of screwing members. One or more than one interconnected grooves are concaved on a surface at one side of the fin, and are connected to a liquid inlet junction and a liquid outlet junction on the substrate, respectively. Accordingly, a heat exchange of the liquid between the liquid inlet junction and the liquid outlet junction can be performed through the grooves of fin, so as form a narrow and thin shape of entire heat dissipater, thereby achieving a function of space saving.