In this study, delignified flax fiber was treated with a NaOH aqueous solution. XRD diffractometry and FTIR spectroscopy were utilized to examine the structural transition of the alkali-treated flax fibers. Also, the effect of the hemicellulose removal on the structural change of flax fibers was discussed. XRD measurement revealed that the crystallinity of cellulose I increased at low NaOH concentration (3%) due to the elimination of amorphous hemicellulose. The structural transition from cellulose I to cellulose II occurred in a NaOH concentration range of 12~15%. Considering most hemicellulose in flax fiber is removed at 12% NaOH, it can be assumed that the presence of hemicellulose in flax fiber has a role in preventing the structural change of flax cellulose. IR absorbance ratios ($A_{1370cm}{^{-1}}/A_{2900cm}{^{-1}}$ and $A_{1430cm}{^{-1}}/A_{2900cm}{^{-1}}$) were utilized as a barometer of cellulose I crystalline exhibiting a similar result with the cellulose I crystallinity from XRD. Another absorbance ratio ($A_{895cm}{^{-1}}/A_{2900cm}{^{-1}}$) reflected the cellulose II crystalline showing almost the same trend as cellulose II crystallinity. On the whole, the total crystallinity of flax fiber was increased at low NaOH concentration (3%) and decreased at 12% due to the reduction of cellulose I content and increased again at 15% attributed to the formation of cellulose II content.