Abstract: Being two critical parameters of API thread,the stand-off and the middle diameter have their tolerances interact and condition each other. Following the conventional measurement method,the stand-off is taken as the main controlled index,whereas no direct measurement is made to the middle diameter. Analyzed here in the article are the relationship between stand-off and such elements as middle diameter,taper and ovality. It is concluded that the actual state of the threads can not be fully identified by means of control of the stand-off, which is no good for control of the thread quality. This theory is also verified by relevant testing activities. In addition,the feasibility of measuring the middle diameter is studied,which leads to a proposal that oil casing makers and oil field end-users substitute middle diameter measurement for stand-off measurement.