A partially cross-linked polymer composition obtainable by dynamically heat-treating: 95 to 50% by weight of a resin (A) comprising a defined ethylene copolymer, and 5 to 50% by weight of a resin (B) comprising a two or more-component propylene copolymer containing 0.1 to 20% by mol of units derived from .alpha.-olefins other than propylene, or a blend of the two or more-component propylene copolymer and a propylene homopolymer which can be in the presence of defined amount of an organic peroxide (C) based on 100 parts by weight of the total amount of the resins (A) and (B). A process for producing the polymer compositions is also provided.