The structure formation and crystallization kinetics in crystallization from a disordered melt were investigated for a polyethylene-polyisoprene block copolymer (LEI) having M-n = 3.2 x 10(4) and 53 wt% of polyethylene content and for its blends with the corresponding homopolymers, polyethylene (PE) and polyisoprene (Plp), using synchrotron small-angle X-ray scattering techniques (SAXS) and differential scanning calorimetry (DSC). For LEI copolymer and the blends, no microphase separation structure was observed in the molten state. In the crystalline state of the neat LEI, the first and higher order scattering peaks were clearly observed, in which the intensity of the higher order peaks was considerably strong. This unusual behavior of the higher order peaks was explained by the lamellar insertion model of Hama and Tashiro. From the analyses based on this model and one-dimensional electron density correlation function with a three phase model, the phase structure in the crystalline state of the neat LEI was concluded to be a regular lamellar structure consisting of crystalline lamella of PE block and amorphous layers of PE and Pip blocks. This phase structure was quite different from that reported previously for a polyethylene-polyisoprene block copolymer (HEI) with a higher molecular weight in which HEI crystallized with keeping the microphase separation structure in the melt. For the blends of LEI with Pip homopolymer, the phase structure is affected by the blend composition, while for the blends with PE homopolymer, the phase structure depended on the crystallization temperature as well as the molecular weight and composition of the added PE. The Avrami index was 2-3 for neat LEI, all blends and PE homopolymers. (C) 2009 Elsevier Ltd. All rights reserved.