Constitutive equations of creep in polygonized substructures are formulated based on the composite model of heterogeneous dislocation structure. The suggested set of equations consists of a kinetic equation, which takes into account the elastic and plastic components of strain in hard and soft regions of the structure, and of two evolution equations, which describe the evolution of the local stress in soft regions and of the volume fraction of hard regions chosen as internal variables. As an illustration, the equations are applied to creep and dislocation structure data of α-iron discussed recently from another point of view.