In this paper polyester-based biocomposites with different amount, with and without hot-water pretreatment, as well as particle size of Miscanthus × giganteus were prepared. Mor-phology of prepared biocomposites was analyzed by means of scanning electron microscopy (SEM). Flexural modulus of biocomposite samples was determined using standard testing procedures. Results were compared for different amounts, particle size and pretreatment of Miscanthus fibers. Best results of flexural modulus were achieved for biocomposites with 15 wt. % of non-hot-water pretreated Miscanthus, with particle size below 200 µm.