© 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.The oxidative thermal stability of plant-based microcrystalline flax fiber was developed by incorporating diammonium phosphate (DAP), boric acid, and urea (in brief DAP-BAU) followed by a multistep thermal oxidation process. By utilizing a set of measurements, including X-ray diffraction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and infrared (IR) spectroscopy analysis, the impact of DAP-BAU inclusion on the thermal stability of flax fibers was studied. The findings of IR spectra and X-ray diffraction analysis revealed that the dehydrogenation and dehydration processes cause a progressive and steady loss of inter- and intramolecular H-bondings. Infrared spectra also showed the development of C = C double bonds associated with the crosslinked ladder structure formation. DSC and TGA findings revealed that DAP-BAU incorporation boosted thermal stability by generating ladder-like structure formation and restricted the development of volatile by-products by inhibiting the fundamental hydroxyl groups with increasing oxidation time. The overall findings of this study confirm that DAP-BAU incorporated and 125 min stabilized (at 245 °C) flax fibers attain complete thermal stability and are ready for utilizing in the subsequent carbonization and activation stages in activated carbon fiber manufacturing.