Two kinds of ethylene-alpha-octene copolymers (ethylene-alpha-octene block copolymer, OBC, and ethylene-aoctene random copolymer, ORC) and their composites with MWCNTs were investigated by means of various solid-state NMR techniques. C-13 MAS NMR revealed that there were only orthorhombic crystals in OBC composites, which resulted in stronger volume exclusion effect towards MWCNTs than that in ORC composites. The degree of microphase separation is quite different for the OBC and ORC composites as revealed by 2D WISE NMR experiment, which also indicated that MWCNTs selectively localize in the immobilized interphase between crystalline and amorphous mobile phases, acting as a "compatibilizer" in OBC-based materials. Based on the spin diffusion results, the thicknesses of the interphases were estimated to be 10.4 nm for OBC and 5.4 nm for ORC respectively. It is also found out that the interphase thickness and the dimension size of MWCNTs were at the same nanoscale for OBC. Consequently, compared with ORC composites, a uniform dispersion of MWCNTs can be achieved just by utilizing OBC copolymer with blocky architecture and larger interphase thickness, and then much lower conductivity percolation threshold can be obtained in OBC composites. (C) 2017 Elsevier Ltd. All rights reserved.