Abstract: The fracture initiation mechanism of refracturing was investigated by discussing rock failures.Both the objective and subjective effects such as distribution of natural fracture,geostress field,mechanical characteristics,and physical properties of rock,well-completion methods,injection/production and fracturing technology upon rock failures were taken into account.As a result,the theoretical criteria for fracture reorientation were given,namely the critical bottomhole pressure and the conditions of geostress field and mechanical parameters of rock.Based on the above criteria,a typical well was analyzed by use of the refracturing data of Chang 6 formation in Wangyao section of Ansai oilfield.It was shown that the fracture reorientation was not equivalent to that of the geostress field and it could also be achieved by proper fracturing technology or approaches even if the geostress field remained constant.