Cyclic hardening behavior of stainless steel under various loading paths is modeled by using the small-strain, isotropic theory of viscoplasticity based on overstress. The cyclic hardening is modeled by postulating a growth law of an internal variable which is equivalent to isotropic hardening of yield surface plasticity. The growth of the hardening variable is formulated in a discrete way by using changes in the direction vector of the inelastic strain rate and the concept of irreversible plastic strain. The irreversible plastic strain is given as a function of variables representing amplitude, nonproportionality, and the history of the loading path. This formulation does not use separate growth laws for proportional and nonproportional loading. They are handled by a difference of the direction vectors of inelastic strain rate before and after it changes its direction. The capability of the model is demonstrated by numerical experiments for various loading conditions.