This paper suggests a theoretical model to estimate Young's modulus of green composites reinforced by natural fiber twisted yarns considering a migration structure. First, we made fully green composites using biodegradable resins and ramie twisted yarns to develop a biodegradable material, and then investigated the effect of yarn twist on mechanical properties of green composites. Next, the migration structure of twisted yarn was taken into account and the relation between twist angle and Young's modulus was discussed by proposing a new reduced stiffness for twisted yarn, in which a twisted yarn was regarded as an orthotropic material with an off-axis angle. The proposed reduced stiffness was verified through comparison between theoretical and experimental results.