Abstract: By theory and numerical simulation, influences of direction and location of defects on expanding fracture of a metal cylinder were analyzed and a new explanation was preasented by the microdefect methods. A computational model for the cylinder containing microdefects was adopted to explore influences of microdefects on expanding fracture of a metal cylinder. Computational results show that microdefects can quicken the radial extension of the cylinder and mutual impenetration of shear extensions. Analysis indicates that the experimental expanding fracture strain is different from the fracture strain as a material parameter offered to the computational program during rupture of the cylinder along its wall thickness directoion, and the two strains have a certain relation.