Abstract: In traditional design of string pipe, the examination is conducted only upon the connection strength of threeds of tubing and casing and the check on leakage resistance of threads is neglected that may lead to the failure of some oil and gas productive wells by the reason of thread leak off. In reference of the characteristics and influencing factors of the thread seal, this paper studied the determination method of leak off resistance of tubing and casing threads and the selection of thread type for design job of string pipe.