Limiting plant fibre damage during industrial processing is challenging for biobased composite development due to plant cell wall sensitivity to temperature and shear. This study developed an innovative fluorophore grafting method to indirectly monitor flax fibre morphological damage during twin-screw extrusion in a poly-(caprolactone) matrix. Results showed the fluorophore was strongly degraded after severe process conditions, with-87.1 % and-72.2 % changes in fluorescence intensity at 240 degrees C and 300 rotations per minute, respectively, compared to native fluorescence. The fluorescence intensity correlated well with the fibre aspect ratio and length, making it a reliable indicator of the fibre's morphology degradation.