Abstract: The paper first briefly introduces the sizing process for quenched/tempered blank tubes for oil casings originally used in the facility of Chengdu Seamless Steel Tube Co.,Ltd. Based on the O.D.expansion rule learned via repeatedly measuring of O.D.of N80 casing blank tubes before and after quenching/tempering,proper tolerance of O.D. control has been set up.Also discussed are the two processes for the tubes,i.e. post-quenching/tempering hot-sizing process and non-post-quenching/tempering hot sizing process.The operational results show that without post-quenching/tempering hot-sizing treatment,it is allowable for producing ordinary hi-strength oil casing pipes of N80 and higher grades up to API Spec.5CT. However,to produce oil casing pipes with high O.D.accuracy and to expand the product-mix,it is still a necessity to get the pipes undergo hot-sizing after quenching/tempering.