The present invention is a polar group-containing olefin copolymer having excellent adhesion properties to metals or polar resins and excellent compatibility therewith. A process for preparing the copolymer, a thermoplastic resin composition containing the copolymer, and uses thereof are also described. The polar group-containing olefin copolymer comprises a constituent unit derived from an .alpha.-olefin of 2 to 20 carbon atoms, and a constituent unit derived from a straight-chain, branched or cyclic polar group-containing monomer having at the end a polar group such as a hydroxyl group or an epoxy group and/or a constituent unit derived from a macromonomer having at the end a polymer segment obtained by anionic polymerization, ring-opening polymerization or polycondensation. The polar group-containing olefin copolymer and the thermoplastic resin composition containing the copolymer are used for films, sheets, modifiers, building/civil engineering materials, automobile exterior trim, electric/electronic parts, coating bases, compatibilizing agents, etc.