In this study, we compared the stress and tensile strength of the hair treated with oxidative permanent dye with those of virgin hair. We investigated the fine structure of the hair section after the tensile test using scanning electron microscopy. The tensile strength of the virgin hair was measured as $14.66;g/cm^2$, tensile energy was $108;erg/cm^2$, and the maximum stress was 146.64g. Those of the dyed hair were $13.69;g/cm^2$, $89.62;erg/cm^2$ and 136.90 g, respectively. The differences in the tensile strength, the tensile energy and the maximum stress were $-0.97;g/cm^2$, $-18.38;erg/cm^2$, -9.74 g, respectively, which showed that the dyed hair had less elasticity and strength than the virgin hair. In the scanning electron microscopy investigation of the damaged hair after the tensile test, lift-off of the cuticle outer layer were shown in both virgin hair and dyed hair, which was more severe in the dyed hair than the virgin hair. Adjacent cuticular cells of the cuticle layer were separated by the destruction of intercellular membrane complex. The macrofibrils were exposed and separated from the cortex torn by tensile strength.