Flax fabrics composed mainly of cellulose were subjected to heterogeneous acetylation after four different pretreatments. The progress of the chemical modification (acetylation) was assessed by ATR-IR. When flax fabrics were pretreated with alkaline solution for 1 h, natural twists of flax fiber disappeared with the change in cross-section to round shape, and its tensile strength decreased up to 50%. Acetylation rate of alkaine-treated flax fabrics treated with glacial acetic acid (Method I) was faster than that of fabrics without glacial acetic acid (Method II). In Method III and IV, alkali treating times were varied from 0 to 30 min to reduce the decrease in tensile strength of flax fabrics without subsequent acetic acid treatment. Acetylation reaction of flax did not occur without alkaline treatment (Method III). In the case of the alkaline treatment for 1 min, the acetylated flax fabrics showed similar tensile strength value to original flax fibers (Method IV).