Gradient structures have excellent mechanical properties,such as synergetic strength and ductility.It is desirable to reveal the connection between the gradient structure and mechanical properties.However,few studies have been conducted for materials with heterogeneous dislocation distribution.In the present study,we use the discrete dislocation dynamics (DDD) method to investigate the effect of dislocation density gradient on the elastoplastic behavior of single crystals controlled by source activation.In contrast to the intuitive expectation that gradient structure affects the mechanical properties,the DDD simulations show that the elastic moduli and yield stresses of three gradient samples (i.e.,no gradient,low gradient,and high gradient) are almost identical.Different from the progressive elastic-plastic transition in the samples controlled by Taylor hardening (i.e.,the mutual interaction of dislocation segments),the flow stresses of source activation ones enter into a stage of nearly ideal plasticity (serrated flow) immediately after yielding.The microstructure evolution demonstrates that the mean dislocation spacing is relatively large.Thus,there are only a few or even one dislocation source activated during the plastic flow.The intermittent operation of sources leads to intensive fluctuation of stress and dislocation density,as well as a stair-like evolution of plastic strain.The present work reveals that the effect of dislocation density gradient on the mechanical response of crystals depends on the underlying dislocation mechanisms controlling the plastic deformation of materials.