A method for assembling multilayer packages of semi-conductor elements comprising double molding of the multilayer structure. The method comprises the steps of primarily molding inner leads of a lead frame, secondarily molding the inner leads to form a desired package, and performing in turn die bonding, wire bonding, trimming and forming processes. The double molding process is performed by using an inexpensive molding compound, thereby obtaining packages having a structure equivalent to that of expensive ceramic packages. Accordingly, the manufacture cost of packages is inexpensive and the assembling process is simplified.