Natural fibers are used as reinforcement for polymer matrices due to their characteristics of being renewable, cost-effective, lightweight, and naturally biodegradable. To replace glass fibers with natural fibers and use them in structural polymer composites for vehicle applications to reduce the carbon footprint, the mechanical properties, specific strength, and modulus of the natural fibers need further improvement. Supercritical CO2 is used to dye fabrics due to its high solubility. In this study, flax fibers were treated using supercritical carbon dioxide (CO2), Nitrogen (N2), Argon (Ar), and Titanium dioxide (TiO2) nanoparticles. After treatment at 28 MPa, 60 ℃ for 24 h by supercritical CO2 and TiO2, the modulus and strength of the thin technical fiber measured in tensile tests were respectively improved by 17% and 53%. The trend in modulus enhancement from thin technical fibers remains similar to the trend reported earlier for thick technical fibers. The real-time treatment process was monitored through the transparent window on the pressure vessel. The mechanisms of fiber fibrillation and extraction were confirmed by tracking the morphology changes of single fibers.