A flexible monofilament of at least 0.08 mm in diameter excellent in transparency, high in impact strength and durability and showing a tensile strength of not less than 30 kg/mm.sup.2 and a modulus of elasticity of 60 to 120 kg/mm.sup.2, which is essentially composed of a vinylidene fluoride-based resin, a polymeric substance and a polyester plasticizer and is prepared by the process comprising the steps of:melt-spinning of a resin composition comprising essentially of 100 parts by weight of said vinylidene fluoride based resin selected from homopolymers of vinylidene fluoride and copolymers thereof; 5 to 15 parts by weight of said polymeric substance selected from homopolymers of methyl acrylate, copolymers comprising at least 85% by weight of methyl acrylate units and at most 15% by weight of isobutylene units and mixture thereof; and 2 to 7 parts by weight of said polyester plasticizer selected from the group consisting of polyesters of 1,100 to 5,000 in molecular weight obtained from an alcohol of 4 to 7 carbon atoms and a carboxylic acid of 4 to 8 carbon atoms and polyesters of 1,500 to 4,000 in molecular weight obtained from a dihydric alcohol of 2 to 4 carbon atoms and a dicarboxylic acid of 4 to 6 carbon atoms, thereby obtaining an unstretched monofilament,stretching the thus obtained unstretched monofilament in a heating medium, andsubjecting the thus stretched monofilament to relaxation thereby obtaining said flexible monofilament.