The modulus of fiber is increased with the increase of degree of crystallinity of fiber. The crystallinity i.e. the regular arrangement of polymer  molecules in fiber matrix improve the inter polymer forces of attraction which results higher strength of fiber. The higher strength of fiber shows the higher value of Young's modulus in the stress-strain curve of fiber.    

Thank you for your comment dear Rahman Bhuiyan

but , can you give me a comment relate to the increase in crystallinity with the decrease in young's modulus