A powder coating composition comprising1. an unsaturated polyester which is prepared by reactingA. terephthalic acid or its lower alkyl ester, andB. an .alpha.-olefinic dicarboxylic acid, andC. optionally an organic acid having at least three carboxy groups in the molecule or its functional derivative, andD. an aliphatic or alicyclic glycol; and2. an organic peroxide and a molecular terminal cross linking or bridging agent, said powder coating composition being able to form a film having excellent properties, such as excellent gloss, hardness, impact strength, heat resistance, weatherability and corrosion resistance.