tMechanical pretreatment is an effective process for chemical or biochemical conversion of woodybiomass. The deconstruction features of the wood cell wall play an important role in its chemical or bio-chemical processing. In this work, we evaluated the wood cell wall fracture in the early stage of mechanicalpretreatment process conducted with various initial moisture contents. Electronic microscopy (i.e., SEMand TEM) and confocal laser scanning microscopy (CLSM) were used to visualize the cellular structurechanges due to cell wall fractures.Results reveal that the enzymatic digestibility of micronized wood produced from different initialmoisture contents was improved by 2–6 folder than that of the raw material. The types of cell wallfractures after mechanical pretreatment were distinguished by the initial moisture contents of wood. Inwood samples with lower moisture content, interwall fracture occurred predominantly at the middlelamella region, while intrawall fracture occurred primarily at inner cell wall layers, with sever breakagein wood fibers for high moisture content samples. Differences in the distribution of surface chemicalcomposition also resulted from different cell wall fractures. Lignin preferentially covered the fracturesurface of low-moisture content samples, while carbohydrates were more predominate in high-moisturecontent samples. These morphological and structural alternation contributed to improving enzymaticdigestibility of micronized wood.Findings from this study demonstrate how mechanical pretreatment modifies the fracture features ofwood cell wall for further chemical/biochemical reactions.