The flax fiber was treated with various concentrations of NaOH solution for the purpose of removal of hemicellulose. The weight loss, which was mainly due to the decrease of hemi cellulose content, was affected by the conditions of NaOH treatment, such as temperature, time and concentration. The crystallinity and birefringence were increased with the NaOH concentration up to 10c1e. Also, the fiber bundles or fibrils were separated more and more as an increase of concentration upon the examination of fiber surface. The critical concentration was observed at around 12% NaOH concentration, in which the crystallinity and birefringence values were very low. Some cracks were appeared on the fiber surface, indicating that the fiber damage occurred. The presence of lignin and hemicellulose as well as the crack formation and fiber bundle separation are important factors for the breaking strength of flax fiber. Moisture regain, water remention value, degree of swelling and the amount of dye adsorption were also strongly related to the structural characteristics caused by NaOH treatment. These physical values were high for the fiber treated with NaOH concentration more than 1071.