This work focuses on the design and fabrication of a continuous filament made of flax yarn and bio-based thermoplastic polymer. For this purpose, the sheath-core process has been explored by using an extrusion laboratory line equipped with a wire coating die to manufacture a flax core coated with PLA. The polymer properties were investigated (thermal transition, mechanical properties, melt viscosity) to link the polymer properties, the process and the properties of the resulting filament. The filament diameter decreases and surface defects appear when the pulling speed increases. The mechanical characterization of flax/PLA filaments demonstrates a higher elastic modulus, higher stress at break and lower strain at break as compared with pure PLA filaments. Such PLA coated flax filaments are a relevant option to widespread the use of biobased composites.