The melt rheology of a commercially available tetrafluoroethylene/hexafluoropropylene copolymer, which is known as Teflon FEP copolymer, was studied to examine the effect of pre-thermal history during sample preparation conditions on dynamic shear and uniaxial elongational measurements. The first experimental series includes the sample preparation under hot press at 320 degreesC followed by a rapid cooling. The master curves were successfully obtained at 300 degreesC from the time-temperature superposition principle. The loss modulus G" was found to be proportional to the angular frequency in a double-logarithmic plot toward 0.01 (rad/s), while the slope of the storage modulus G' did not become 2. The elongational viscosity as a function of time under constant strain rates showed weak strain-hardening, which was enhanced with larger strain rates. The second experimental series contain three kinds of samples with different pre-thermal history to control rheological properties. All samples were hot-pressed at 320 degreesC followed by a rapid cooling to room temperature for the sample A and a slow cooling for the sample B and C. The dynamic shear and elongational measurements were performed at 270 degreesC for all samples, which were heated from room temperature for the sample A and B, but heated up to 280 degreesC and cooled down to 270 degreesC for the sample C. The distance between G" and G' become narrower in the order of the sample C, B, and A. In the same order, unexpectedly, the strain-hardening in the elongational viscosity measurements became the strain-softening. These unusual properties were discussed from a residual crystallinity.