Uniaxial elongational flow behavior was examined for polystyrene(PS)/styrene-butadiene-styrene(SBS) triblock copolymer by using a Meissner type rheometer under uniaxial elongational flow at constant strain rates. The mixtures of PS and SBS were prepared by means of a solution blending method. Pure PS did not exhibit strain-hardening. Elongational viscosity was quite similar among the mixtures of PS/SBS without heat treatment. However, large enhancement of strain-hardening was observed for PS/SBS blends treated at temperatures higher than 220 °C, and the magnitude of enhancement reached ten times or more. To analyze the cause of the enhancement of strain-hardening, we examined pure SBS itself using a combination of uniaxial elongation and FT-IR spectroscopy. Change in uniaxial elongational flow behavior, i.e., from strain-softening to strain-hardening, was observed corresponding to heat treatment temperatures. From FT-IR spectroscopy of SBS treated at various temperatures, the correlation between enhancement of strain-hardening and crosslinking of butadiene was obtained. From shear stress relaxation measurement of PS/SBS(90/10) treated at 260 °C for 60 min, convergence of relaxation modulus was observed in a strain range of γ ≥ 2 at long times. Thus, the increase of strain-hardening of PS/SBS blends treated at high temperatures is likely to result from affine deformation of crosslinked SBS component.