The methods for determining the cyclic strengths of metals under low-cycle and high-cycle fatigue are different, since the mechanisms of failure and the extent of the presence or absence of plastic deformation differ for each type of fatigue. The aim of this study is to develop united models for both low-cycle and high-cycle fatigue life prediction. We propose that the relevant relaxation and damage processes are considered and it on different types of metals is tested. In this paper, the cyclic deformation of materials is considered using the proposed model with regard to two processes: stress relaxation and damage accumulation kinetics. Proposed approach allows us to study the united fatigue curves of materials regardless of the chosen type of fatigue (low-cycle fatigue, high-cycle fatigue, giga-cycle fatigue). Fatigue life curves under staircase strain loading and symmetrical sinusoidal strain/stress loading are predicted in this study. A simple numerical scheme for the model is successfully applied to various materials under various types of loading, since the relaxation–kinetic model is phenomenological in nature.