Nanotwinned copper has been shown to greatly improve the yield strength while maintaining good electrical conductivity. It has great potential to be incorporated in the very-large-scale integration of Cu interconnect technology. The influence of stress/strain on nanotwin formation is studied using first principles calculations of the total crystal binding energy. Under biaxial stress, the total energy of strained Cu can be larger than that of strain-relaxed periodic nanotwinned Cu. We propose that, during pulse electrodeposition of Cu films, highly strained Cu can undergo recrystallization and grain growth to relax stress and form strain-relaxed nanotwins.