Morphological effects on molecular mobility have been studied for solid ultrahigh molecular weight polyethylenes (UHMW-PE) crystallized from the melt and from solution or during polymerization. On the basis of transmission electron microscopic (TEM) observation, a crystalline domain structure was well identified for nascent UHMW-PE powders, which is quite different from regular lamellar stacking for solution-crystallized samples and the usual spherulites for melt-crystallized samples. Nuclear magnetic resonance (NMR) results showed that the amorphous chains between these crystalline domains in nascent powders were constrained, as well as those sandwiched between stacked crystalline lamellae for the solution-crystallize sample. Also, the existence of three regimes was recognized in the relaxation behavior of the crystalline phase, as revealed by H-1 pulse NMR measurements. In process 1 (heating from room temperature), activation of molecular motion at the boundary between crystal/amorphous regions takes place. During process 2 (above the critical temperature of 60-90 degrees C), the crystallinity increases with the acceleration of the entire molecular motion caused by sliding of molecular chains in the crystalline region. Further raising the temperature (process 3) leads to the start of sample melting. These relaxation mechanisms suggest that the accelerated molecular motion in the crystal/amorphous boundaries initiates following lamellar thickening without passing the melt state.