The effects of thixoforming defects on the stress-strain curves of aluminum structural part were quantitatively investigated. The A357-MHD billet was thixoformed to manufacture a shock tower part of a small electric vehicle. All tensile specimens were machined from only a shock tower product to minimize the effects of different processing conditions. Tensile tests were performed under the almost same conditions. The fraction of solid and liquid phase, the sizes and area fractions of defects in the specimens were measured and analyzed by an image analyzing program using OM and SEM micrograph. The fracture strain values of the stress-strain curves were greatly affected by amount of defects. However, the analyses with defects existed in the area of gauge length did not show a clear explanation about the relation with the fracture strain values. The measurements of defects in the fractured surface represented more plausible results about the correlation of forming defects and fracture strains.