In a method for forming a package, a package with the shape of a container is formed of a cardboard blank, the package having a bottom and side walls extending upwards from the bottom. At the end stage of compression molding, a rim of molding material extending sideways is molded to the upper part of the side walls when the mold is kept closed in such a way that a mold cavity corresponding to the rim is formed by moving a counter surface inside the mold backwards from a surface opposite to it, after which the molding material is introduced into the mold cavity.