a copolymer comprising chlorotrifluoroethylene and 0 . 01 to 1 % by mole of perfluoro of the formula : r . sub . f -- o -- cf ═ cf . sub . 2 wherein r f is a c 3 - c 6 perfluoroalkyl group , and having a melt flow rate of 0 . 01 to 200 × 10 - 3 cc / sec ., which has good mechanical strength and sufficiently high melting point .