The ultra-high modulus state of polyethylene (UHMPE) can be obtained by various methods which produce a n extensive preferred orientation of the molecular chains in the direction of the fiber axis. The tensile modulus is, however, nearly independent of the degree of preferred orientation, in contrast to other high modulus fibers a s , e.g., poly(pphenyleneterephtha1amide) o r carbon fibers. This is explained by the effect of trapped loops in the fibrillar structure of UHMPE which a r e formed during the deformation process. Varying the crystallization conditions for the starting material leads to changes of the mechanical properties of UHMPE produced by hot drawing which can be explained by the proposed structural model. Recent small-angle neutron studies ( 10,ll) have shown that the large-scale conformation of the molecules is not changed significantly upon crystallization from the melt which means that there should be a considerable interpenetration of the molecules in melt-crystallized material. On drawing this material with high draw ratios entanglements will be formed which produce a s e r i e s of extended chain segments interconnected by trapped loops. This i s considered to be the essential proc e s s for the formation of a fibrillar structural component which is necessary f o r a high tensile modulus.