Abstract: The uniaxial tension with different degree of pre-deformation and baking treatment were carried out on ultra-low carbon steel, and the effect of degree of pre-deformation on the baking hardening property of the test steel was studied, and the dislocation morphology in the test steel with different degree of pre-deformation was observed by the transmission electron microscopy. Results show that when the degree of pre-deformation increased, the BH value increased in the degree of pre-deformation below 4%; declined in the range of degree of pre-deformation from 4% to 8%; and increased again between 8% and 15%. And the maximum and minimum of BH value were found in the degree of pre-deformation of 4% and 8%, respectively. And the dislocation density in 4% degree of pre-deformation was higher than that in 8% degree of pre-deformation.