Generally, interfacial adhesion between natural fiber and biodegradable resin can be improved by alkali treatment. The effect of alkali concentration of treatment on fatigue property of natural fiber as reinforcement should be examined to assure the use of structural material using green composite. The purpose of this study is to examine fatigue properties of 1% and 15% alkali treated natural fiber. Jute fiber was used as specimen. Alkali treatments were conducted in distilled water with NaOH 1% and 15%. X-ray diffraction measurement of alkali treated jute fiber was conducted. Fatigue tests and damage observations of non and alkali treated jute fibers were conducted. As a condition of fatigue test, the stress ratio was 0.1 and the frequency was 10Hz. As a result, following conclusions were obtained. Peaks of diffraction intensities of 1% and 15% alkali treated jute fiber appeared at 22.8° and 22°, respectively. This phenomenon indicated that cellulose in constitution materials of 15% alkali treated jute fiber changed cellulose I to cellulose II. Fatigue strengths of non, 1% and 15% alkali treated jute fiber decreased with an increase of number of cycle. Fatigue strengths at 10 6 cycles of non, 1% and 15% alkali treated jute fibers were 280MPa, 320MPa and 181MPa, respectively. The number of crack in surface of jute fiber at 10 3 cycles increased with an increase of alkali concentration of treatment. Therefore, fatigue property and damage of jute fiber was affected by alkali concentration of treatment.