A novel in situ measurement system for evaluating molecular mobility during uniaxial drawing of polymeric materials was established by introducing drawing and stress-detecting devices into a H-1 pulse nuclear magnetic resonance spectrometer. In this study, we analyze the changes in molecular motion of amorphous chains during melt-drawing of ultra-high molecular weight polyethylenes (UHMW-PEs) with different molecular weight distributions. In the initial stage of drawing, a three-component resolution was possible for the relaxation decay curve, which includes "rigid", "intermediate", and mobile" amorphous components. The quality and quantity changes in these components demonstrated that this intermediate amorphous component could be regarded as the index of the change in molecular entanglement characteristics during the orientation of PE chains.