Abstract: Wood rheology is studied mainly focusing on the deformation law and mechanisms of wood under the conditions of stress/strain, temperature, humidity and other circumstances,and its main research field is wood viscoelasticity. The real load-bearing structure of wood is the cell wall, and the structure and chemical composition of cell wall have a significant influence on its viscoelastic behavior. Therefore, having a good comprehension on cell wall structure and viscoelastic properties is of great significance to realize the high efficiency design of wood fiber/plastic composites and pulp and paper manufacturing process. In this paper, the cell wall structure was described from the cell wall S₂ layer and the chemical composition. This paper summarized the influence of S₂ layer microfibril angle and chemical composition on wood cell wall viscoelastic behavior, and the mechanisms of action were explained at the molecular level. In addition, this paper summarized the application of dynamic mechanical analysis (DMA) and nanoindentation techniques on the studies of the relationships between the wood cell wall layer structure and the viscoelasticity. The viscoelasticity of wood cell wall is influenced by the complexity of cell wall structure, diversity of chemical composition and external environmental conditions and other factors, and there are also some interactions among these factors. Therefore, suggestions for future research should be focused on the following aspects:1) To clarify the molecular mechanisms of microfibril orientation and proportion of cellulose crystalline and amorphous region during the growth of wood cells. 2) To illuminate spatial organization arrangement of Matrix in the S\-2 layer of wood cell wall and the expression of the mechanical behavior of the interaction between microfibril and Matrix. 3) To reveal the influence mechanism of content and types of hemicellulose and lignin on cell wall viscoelasticity. At the same time, the external environmental factors (temperature, humidity) and load types (static/dynamic, tension/compression/bending) were brought into the research system to interpret mechano-sorptive creep behavior law and its responsive mechanisms of cell wall, which is under the combined action of "thermo-hydro-mechanical". 4) To utilize various testing techniques, and to introduce the research method and theoretical models of the related disciplines, such as the finite element method and the research method of the composite materials, with the aims to build physical and mathematical models which can explain the viscoelastic properties of wood cell wall.

Key words: wood, cell wall structure, viscoelasticity, rheology