The interfaces introduced in metals by heterostructural design play crucial roles in mechanical behaviors. Here the effect of gradient interfaces on mechanical behavior was investigated in a laminated Cu–30Zn sample composed of coarse-grained and ultrafine-grained layers. Tensile tests revealed a superior strength-ductility synergy with extraordinary strengthening and work hardening. By combining the measurements of height contour and strain distribution using digital image correlation, the development of strain gradient was detected in the near-interface zone during tension, which was caused by the mechanical incompatibilities across interfaces and the synergetic constraint between layers. The intensity of strain gradient in the near-interface zone increased with tensile strain, which was accommodated by the accumulation of geometrically necessary dislocations, thereby resulting in extra back stress and dislocation strengthening. •Gradient interface in CG/UFG laminate leads to extra strengthening and hardening.•Gradient interface accommodates the strain incompatibility by accumulating strain gradient.•The development of 3D strain gradient around interface is quantitatively visualized.•GNDs accumulation around interface is theoretically derived and experimentally verified.•Gradient interface leads to extra dislocation and back stress strengthening.