The equation of mechanical states in the description of high density polyethylene creep under unsteady conditions of static and cyclic loading under plane stress state was built on the basis of experimental results. It takes into account momentary-elastic,viscoelastic, viscoplastic and momentary-plastic and strain components using the conditions of constant volume of material. Similarity conditions of stress and strain deviators apply them according to the equation. Introduction parameters are functions ofthe form of the stress deviator, as well accounted for in this equation. Obtained from the calculation of creep strain in stationary and non-stationary analytical and static loading is described satisfactorily by the proposed equation that follows from a comparison with the experimental data.