Abstract: It is necessary to study how to determine the optimum of the total number of effective turns of the round jointing thread of oil casing pipes.
In this Paper, the equation of the load of the jointing thread is obtained by analysing elastic of the round jointing thread of the casing string under stretching stress. The optimal number of turns of the round jointing thread of oil casing pipe is deduced by means of numerical calculation of the dimension parameters of Chinese-made casing pipes and casing collar in technical manual (YB690-70) and by analysing the load distribution feature in jointing therad.