© 2019 Elsevier B.V. The dominance of microstructure on the deformation behavior of a nanowire is a fundamental issue in the application of nanodevices. Through the use of molecular dynamics simulations, the deformation and breaking behavior of convex and concave copper nanowires under uniaxial tension has been studied. Statistical analysis of a large number of independent samples with different microstructure has been carried out and compared to the single-crystal copper nanowire. Without neither the convex nor the concave nanostructure, failure tends to occur at three preferred locations within the wire, although it can occur anywhere. The dislocations that form initially are found to emerge randomly at the edge of the nanowire. For convex nanowires, the initial dislocations are generated randomly outside the convex region with a breaking position distribution otherwise similar to that of single-crystal nanowires. The results indicate that the convex structure may strengthen the nanowire and reduce the breaking possibility. An axial stress analysis shows a stress valley on the profile, revealing the reason for the breaking position distribution. On the contrary, the concave nanowire displays a different character in the mechanical property. From the atomic configuration, it is observed that the edge is the main source of the dislocations, which significantly increase the probability of the breaking position occurring there. The axial stress analysis proves that a stress hump is generated by the concave microstructure, highlighting the influence of the microstructure. The results are in line with what one would expect intuitively and what is seen in macroscopic structures – failure is more likely within thinner sections as stresses are higher there due to the reduced cross-sectional area. These results point towards a quantitative estimation of the microstructure effect on the breaking distribution of a nanowire.