The structural evolution of poly(ether-b-amide12) (PEBAX$^{(R)}$) elastomers was studied by using wide angle X-ray scattering (WAXS) at a synchrotron radiation source during the uniaxial stretching. The changes in the crystal structure of the nylon 12 were tracked during the stretching of the P6333 and P2533 compressed films, which represent a hard rubber and a soft elastomer, respectively. The nylon 12 chains of both samples favored the ${gamma}$-phase in the unstrained state but their responses to the external drawing force were different. For the P6333 film containing small amounts of the soft segments, the applied force dilated the 001 crystalline lattice plane of nylon 12 which is perpendicular to its main chain and the ${gamma}$ form was transformed into the ${alpha}$" form when the applied force was high enough. However, for the P2533 containing large amounts of the soft segments, the lattice dimensions of the nylon 12 crystals did not change with the external drawing force and the transient ${alpha}$" form was not observed. These results might suggest that the transfer of the external force to the crystal was difficult for P2533 due to the chain relaxation of the soft segments.