Several criteria for fretting wear behavior evaluation have been established since the proposal and establishment of the fretting loop concept. In this article, system deformation and system deformation ratio were defined. In addition, the fretting running conditions were distinguished from the evolution of system deformation with fretting cycles during fretting wear tests under different applied displacements and loads. In the gross slip regime, the system deformation was independent of the applied displacement and increased as the load increased, whereas in the partial slip regime, the system deformation was independent of the load and increased with the applied displacement. Furthermore, a linear relationship between the system deformation and the applied load in gross slip regime was found for the first time. Based on this linear relationship, the system deformation ratio can forecast the running regime with a given load and displacement. For the titanium alloy fretting pairs studied in this article, the fretting wear was found to run in the gross slip regime if the system deformation ratio was smaller than 0.9. Based on these observations, the system deformation ratio exhibited applicability in assisting the mechanical design of equipment suffering from fretting wear.