Disclosed herein is a low shrinkable unsaturated polyester resin composition consisting of (A) 20-70% by weight of an unsaturated polyester, (B) 28-60% by weight of a monomer polymerizable with the above unsaturated polyester, and (C) 2-20% by weight of a block copolymer mixture. The mixtures of the monomer (B) and the block copolymer mixture (C), and a mixture of the polyester (A), the monomer (B), and the block copolymer mixture (C) are in a non-aqueous dispersion state. The block copolymer mixture is obtained by polymerizing either one of the monomers defined below in (a) and (b) with a polymerization initiator a polymeric perioxide represented by the formula: ##STR1## in which R.sub.1 represents a divalent branched hydrocarbon group having 10-30 carbons, and R.sub.2 and R.sub.3 represent a hydrogen atom, a hydroxyl group or an alkali metal, and the average polymerization degree n is 2-40, and is used to obtain a first polymer having peroxy bonds in its molecule, and block-polymerizing the first polymer with the remaining monomer (a) or (b) which has not been used in the first polymerization reaction wherein, (a) is a monomer or a monomer mixture consisting of 70-100% by weight of styrene monomer and 30-0% by weight of a monomer polymerizable therewith.(b) is a monomer or a mixture of monomers consisting of 70-100% by weight of C.sub.1-4 alkyl ester of acrylic acid or methacrylic acid and 0-30% by weight of a monomer polymerizable therewith.