In situ tensile straining in a transmission electron microscope was used to investigate the role of twin plane spacing on the deformation and fracture mechanism of pure copper containing a high density of nanoscale growth twins. Real-time and post-mortem observations clearly revealed that twin plane spacing played a key role in determining the operative deformation mechanism and therefore the subsequent crack propagation path. The deformation mechanism transition, which resulted from changes in the twin plane spacing, has implications for interpreting the unusual mechanical behavior of the copper with a high density of nanoscale growth twins.