The micromechanical hardening rules developed by Cuitino and Ortiz [ Modelling Simul. Mater. Sci. Eng. 1 (1992) 225-263 ] and Repetto and Ortiz [ Acta mater. 45, 6 (1997) 2577-2595 ] are modified in order to establish a new approach to the study of PSB formation process. A crystalline FEM theory which incorporates this modified hardening rule is presented. We explore the propriety of the presented hardening rule and FEM theory by comparing the calculated and measured cyclic stress-strain relations of Cu single crystal. As results, the followings are found : 1) The presented hardening rule and FEM theory give a good account of the nature of cyclic stress-strain relation of real f.c.c. crystals qualitatively. 2) The calculation result shows that the secondary slip occurs at the moment Bauschinger energy parameter β _E shows its maximum value. PSB formation begins at about the same time. It is presumed that the occurrence of secondary slip is related to the formation of PSB.