Strength, elongation and elasticity are essential properties with regard to the tensile properties of raw silk thread. The reasonable harmony of the above three physical characters appears to be the specific nature of the raw silk thread and the dynamical functions are determined by the cocoon filament forming the raw silk. From such viewpoint, the author has studied the tensile property of the thread when the cocoon filament is stretched and transformed. The results obtained are as follows: 1) The stretching strength of the inner layer filament is stronger than that of the outer and medium layer ones. 2) The stretching strength of the filaments is influenced by the moisture-heat denaturalization of cocoon layer sericin in the drying process. 3) The more transformed functions by the stretching become, the lower the elasticity index of the cocoon filament was shown. 4) When cocoon layer sericin is denatured by moisture and heat, elasticity index of the filament is decreased.