Crystallization-induced structure fluctuations in the layer thickness direction and disordering of pre-crystallized structure in crystallized microdomain structure that consist of a strongly segregated crystalline-crystalline diblock copolymer, poly(ethylene glycol)-block-poly(perfluorooctyl ethyl acrylate) (PEG-b-PFA-C-8), were investigated. The structure formation process during isothermal crystallization was monitored by in-situ time-resolved simultaneous SAXS/WAXD measurements and thermal analysis. At early stage of isothermal crystallization, three dimensional growth of the PEG lamellar crystals within lamellar microphase-separated morphology induced the volume shrinkage of PEG phase, resulting in the reduction of PEG layer thickness, increase of PFA-C-8 layer thickness and disordering of pre-crystallized PFA-C-8 crystals. At late stage of isothermal crystallization, one dimensional growth, thickening of PEG lamellar crystals induced the increase of PEG layer thickness and reduction of PFA-C-8 layer thickness. The structure fluctuation in the crystallized microdomain structure depends on the crystal growth dimension. (C) 2016 Elsevier Ltd. All rights reserved.