Abstract: The steel grade and thread connection strength specified by API standards cannot meet the requirements of deep and ultradeep wells any more. So far, the casing up to API standard manufactured by various large manufac1ures worldwide have solved this diffi-cult problem. This paper describes some casing failure and the approach for enhancement of casing safety and reliability in deep and ultradeep wells; recommends to select casing with proper steel grade and high collapsing strength and analyzes the sealing structure of special thread for casing with special thread connectors. It presents as well the selection of casing and special thread connector for deep well, ultradeep well, well with high pressure, direc-tional well, horizontal well, thermal recovery well and well with corrosive medium.