According to the microstructure of plant fiber, the plant fiber was considered as an kind of natural macromolecule complex material in which fibril was reinforced material and hemicellulose and lignin were base stock. A mathematical model about microstructure of plant fiber was built according to the laminated board theory. This mathernatical model was used to simulate the effect of angle be- tween fibril and fiber axial direction, modulus and content of hemicellulose, modulus of fibril and thick- ness of fiber S1 layer on fiber isotropic modulus.