<p>The mechanical and microstructural properties of biocomposites produced through hot press compression molding of polylactic acid (PLA), PLA with polybutylene succinate (PLA + PBS), and flax fibers were investigated. The flax mat fibers underwent surface treatment with a sodium hydroxide (NaOH) solution. This treatment induced fibrillation on the fiber surfaces, enhancing the fiber-matrix interfacial bonding and improving the mechanical properties. The density of the biocomposites remained largely unchanged; however, significant variations were observed in their hardness. The tensile strength of surface-treated biocomposites exhibited a substantial increase, ranging from 40 to 60%. The incorporation of PBS resulted in a notable enhancement in elongation-at-break, increasing by 10–40% compared to PLA. In the bending tests, biocomposites containing PBS demonstrated the highest elongation values, whereas those without PBS exhibited the highest bending stress. Moreover, the addition of PBS to the PLA matrix enhanced the plasticization effect, leading to an increase in elongation-at-break. Furthermore, the glass transition temperature and crystallization temperature of flax fiber-reinforced biocomposites (PLA + F) and surface-treated biocomposites (N-PLA + F) increased compared to untreated samples. Additionally, an increase in the melting temperature was observed in all biocomposites, except for the PLA + PBS mixture.</p> Graphical abstract <p></p>