A fluorine-containing polymer substantially comprising (A) at least one fluorine-containing olefin of the formula:CF.sub.2 =CXYwherein X is a hydrogen atom or a fluorine atom and Y is a hydrogen atom, a chlorine atom, a fluorine atom, a trifluoromethyl group, a difluoromethyl group, a C.sub.1 -C.sub.3 perfluoroalkoxy group or a group of the formula:--(CF.sub.2).sub.m COOMwherein m is an integer of 0 to 3 and M is a hydrogen atom, a C.sub.1 -C.sub.4 alkyl group, a sodium atom or a potassium atom but when X is a hydrogen atom, Y is always a hydrogen atom, (B) one or more C.sub.4 -C.sub.8 fluorine-containing dienes, (C) at least one iodine atom and (D) at least one saturated or unsaturated fluoro or chlorofluorohydrocarbon group, the molar ratio of the diene (B) and the olefin (A) being from 1.times.10.sup.-4 to 5.times.10.sup.-2, the iodine atom(s) (C) being from 0.001 to 10% by weight based on the weight of the polymer and the molar ratio of the group (D) and the olefin (A) being from 1.times.10.sup.-5 to 5.times.10.sup.-2, can be cross-linked at atmospheric pressure at a cross-linking temperature in the presence of a cross-linking agent. The fluorine-containing polymer may be prepared by polymerizing (B) with the polymerization reaction product of (A) and an iodide compound.