Abstract: It is identified in R＆D of the Φ339.7 mm P110 hi-strength casing that the variation of the tube end diameter is caused by the stress ageing after completion of the thread processing,resulting in the thread parameters changed gradually.The trend and severity of the said changes are analyzed based on the measurements of the quantitive changes over the elapse of the time of the geometric dimensions and thread parameters of the quenched-tempered and straightened casing.Necessary control actions to ensure the consistency of the casing thread parameters are proposed in accordance with the analysis on the mechanism concerning the stress ageing influence on the steel tube shape.