Fine hemp fibers (cottonized hemp) were processed using steam explosion. The quantification of the defibration rate was performed by image processing. Based on this method, the hemp defibration was optimized using a response-surface methodology based on three-variable central composite design for the production of elementary fibers with low variability. Optimal parameters for the steam processes were as follow: time = 4.1 min, temperature = 191°C. Biomass was impregnated with a solution of NaOH (8 %) before treatment, leading to a defibration rate of 91.2 %, which is producing ≈ 50% fibers with length < 3 mm in good agreement with the experimental data. Damaged fibers originating from the conjugated effect of steam explosion and alkali hydrolysis were also observed