The main focus of this study is to investigate the properties of alkali treated flax fiber reinforced poly(lactic acid)/polycarbonate (PLA/PC) composites. Characterization of these composites was performed by conducting tensile strength, dynamic mechanical analysis (DMA), differential scanning calorimeter (DSC) and scanning electron microscope (SEM) analyses. Tensile strength results revealed that the highest mechanical performance was observed for 2 % sodium hydroxide (NaOH) treated flax fiber reinforced PLA/PC composites. DMA analysis also supported the results of the tensile tests. Tan delta (delta) curves were used for evaluating the fiber-matrix adhesion in flax fiber reinforced composites. The lowest peak magnitude and consequently the best fiber-matrix adhesion were also observed for 2 % NaOH treated flax fiber reinforced composites. DSC results showed that slight changes occurred in glass transition and melting temperature values. Moreover, crystallinity of PLA was affected by the flax fiber reinforcement. The maximum crystallinity value was observed in 2 % NaOH treated flax fiber reinforced composites. Furthermore, SEM analysis showed that 2 % NaOH treated flax fibers have better interfacial bonding with PLA/PC matrix among the flax fibers.