This study presents an advanced numerical model for plant fiber-reinforced composites, addressing a significant gap in predictive modeling for these environmentally friendly materials. Our model describes the complex interactions between cellulosic fibers and matrix in biocomposites, accounting for all major failure mechanisms: matrix softening, fiber breakage, and fiber-matrix debonding. Employing nonlinear plasticity, XFEM, and cohesive zone models, we simulate failure in a unit cell with two fibers and periodic boundary conditions [1]. This approach enables a precise prediction of nonlinear macroscopic behavior in biocomposites. Validated against experimental data, the model accurately predicts tensile and compressive properties of both short- and long-fiber composites. The unit cell method also enables further sensitivity analyses, providing valuable insights into effects such as the softening related to decreased interfacial shear strength and the strengthening impact of longer fibers. This research paves the way for future studies on lignin-based biocomposites [2] and aims to establish a comprehensive link between analytical [3] and numerical modeling approaches for a robust mechanical prediction model for complex biocomposite materials.