High temperature liquid chromatography (HT-LC) and temperature gradient interaction chromatography (TGIC) using a graphite substrate were recently invented to characterize polyolefin microstructures and composition distributions. Their separation mechanisms have been explored by using a series of random copolymers. In addition, the separation mechanism of TGIC for random copolymers was also studied with a high temperature NMR cryoprobe. The proposed separation mechanism for a random copolymer is that the separation is likely based on comonomer content, in other words, the number-averaged ethylene sequence length. Further understanding of HT-LC separation mechanism with polyethylene octene block copolymers is beneficial in order to extend the use of HT-LC and TGIC to a wider variety of polyolefin materials, and to better characterize more challenging microstructures of new materials. With the high temperature NMR cryoprobe technology, it is possible to perform C-13 NMR of polyolefins with a few milligrams of sample. This paper summarizes the NMR study of the materials obtained from preparative scale HT-LC on polyethylene octene block copolymer with a high temperature NMR cryoprobe. The results show unambiguously that the HT-LC separation of polyethylene octene block copolymer is mainly based on the interaction between hard block and the stationary phase, and the strength of the interaction is correlated to the total length of the hard block in the block copolymer.