A method for bonding a heat-conducting substrate and a metal layer is provided. A heat-conducting substrate, a first metal layer and a preformed layer are provided. The preformed layer is between the heat-conducting substrate and the first metal layer. The preformed layer is a second metal layer or a metal oxide layer. A heating process is performed to the preformed layer in an oxygen-free atmosphere to convert the preformed layer to a bonding layer for bonding the heat-conducting substrate and the first metal layer. The temperature of the heating process is less than or equal to 300° C.