In this paper, we study the relationship between the pull-off force and the transition parameter (or Tabor number)as well as the variation of the pull-off radius with the transition parameter in the adhesion elastic contact. Hysteresis models are presented to describe the contact radius as a function of exteal loads in loading and unloading processes.Among these models, we verified the hysteresis model from Johnson-Kendall-Roberts theory, based on which the calculated results are in good agreement with experimental ones.