To obtain copolymers having improved mechanical properties, particularly at high temperatures and, at the same time, excellent optical properties.

The objective heat processable tetrafluoroethylene(TFE) copolymers comprise (A) 0.1-15 mol.% fluorodioxol of the formula (wherein Rf is 1-5C perfluoroalkyl; X1 and X2 are the same or different and each -F or -CF3; and Z is -F, -H or -Cl); (B) 0-15 mol.% perfluorinated monomer selected from hexafluoropropane(HFP), a fluoropropyl vinyl ether of the formula: CF2= CF-OR'f (wherein R'f is 2-4C perfluoroalkyl) and a mixture thereof); and (C) the rest totaling to 100% of TFE (provided that the total amount of monomers (A) and (B) is not more than 20 mol.%).