The effect of NaOH concentration on the mechanical properties of lyocell yarn and fabric has been studied. The breaking strain of yarn increases as the concentration increases. This is due to the contraction of yarn that occurs during swelling. And the initial modulus of yarn decreases as the concentration increases because the lyocell is damaged by NaOH. The tensile strain of fabric increases with the concentration of NaOH due to the contraction occurring during swelling. However, the bending and shear rigidity increases as the concentration increases. This is due to the stiffening of the yarn and increased fabric density that affects the interactive forces between yarns and fibers.