Fiber-reinforced biocomposites made from natural plant fibers and a biodegradable polymer matrix present a high-performance yet more sustainable alternative to conventional synthetic composite materials. Two microstructural aspects governing the composite behavior are the fiber orientation distribution and the fiber-matrix interface behavior, both of which are rather challenging to incorporate into a predictive modeling framework. We use multiscale micromechanics to homogenize the composite stiffness. At the composite scale, any fiber orientation distribution and any fiber aspect ratio distribution may be considered, such that biocomposite microstructures with more or less aligned fibers of different lengths can be suitable represented. Moreover, the often rather weak fiber-matrix bond is dealt with by adopting a spring-type interface model within the theory of continuum micromechanics [1]. A model for the microstructure of the fiber itself is incorporated to homogenize the fiber stiffness based on the known stiffness of its intrinsic constituents (mostly cellulose and lignin) and on geometric features such as the microfibril angle and the lumen porosity [2]. The multiscale model is comprehensively validated by comparing the predicted composite stiffness to experimental results from uniaxial tensile tests, performed on several different plant fiber composites with various orientation distributions. After successful validation, the novel multiscale model is exploited to quantify the stiffness decrease with decreasing fiber alignment and increasing fiber-matrix interface compliance. Increasing fiber lengths (within the typical range of biocomposites) benefits composites with weak interfaces but has little impact on the stiffness of composites with strong interfaces. REFERENCES [1] F. Dinzart, H. Sabar, and S. Berbenni. Homogenization of multi-phase composites based on a revisited formulation of the multi-coated inclusion problem. Int. J. Eng. Sci., 100:136–151, 2016. [2] M. Königsberger, M. Lukacevic, and J. Füssl. Multiscale micromechanics modeling of plant fibers: upscaling of stiffness and elastic limits from cellulose nanofibrils to technical fibers. Mater. Struct., 56(1):13, feb 2023.