By defining the yield stress in a latent hardening test as the first deviation from the elas-tic straight line,the yielding and hardening behavior on a latent system in the positive and negative slipdirection was studied in aluminium single crystals.It is shown that the yield stresses on both the posi-tive and negative latent systems are about equal to or a little lower than the maximum resolved shearstress in the primary test,but much higher than that of the active system.The influence of relativeorientation and prestrain on latent hardening and initial work-hardening in the secondary test was alsoinvestigated,and it was found that there is a considerable effect on initial work-hardening,but noneon latent hardening.With reasonable approximation,a hardening rule for single crystal could be pro-posed from the experimental results,that is,except for the yield stress on the system negative to theactive system that is very low,hardening on the other systems is nearly the same as self-hardening. By defining the yield stress in a latent hardening test as the first deviation from the elas-tic straight line, the yielding and hardening behavior on a latent system in the positive and negative slip direction was studied in aluminum single crystals. It is shown that the yield stresses on both the posi-tive and negative latent systems are about equal to or a little lower than the maximum resolved shearstress in the primary test, but much higher than that of the active system. The influence of relativeorientation and prestrain on latent hardening and initial work-hardening in the secondary test was also investigated, and it was found that there is a considerable effect on initial work-hardening, but noneon latent hardening .With reasonable approximation, a hardening rule for single crystal could be pro-posed from the experimental results , that is, except for the yield stress on the system negative to the active system that is very low, hardening on the other systems is nearly the same as self-harde ning.