Abstract: The disclosure relates to a graft polymerization method to form a fluorine-containing graft copolymer which is useful as a soft and flexible fluororesin intermediate between fluororubbers and crystalline fluororesins. The graft copolymer is formed by polymerizing a fluorine-containing crystalline polymer, e.g. copolymer of CTFE and ethylene, in water in the presence of a reducing agent and a fluorine-containing elastomeric copolymer which comprises peroxy groups and is dispersed in the water to become the "trunk" of the graft copolymer. For example, the elastomeric copolymer is of vinylidene fluoride, CTFE and t-butyl peroxyallylcarbonate. An example of the reducing agent is sodium bisulfite, and optionally and additionally a source of a reducing metal ion such as ferric chloride may be used. This graft polymerization reaction can be carried out at a relatively low temperature, viz. at 20.degree.-50.degree. C.