Abstract: The investigation conducted with the Φ100mm Assel pipe mill line located at the facility of Seamless Steel Tube Co.,Ltd., Wuxi involves different sizes of heavy, medium and light walled tubes. Analysis is made on the gauges, distribution pattern and evolution of both relative and absolute diviatons of gauges of the tubes in different manufacturing steps. The research result shows that in the steps the distributions of the gauge, relative gauge deviation and absolute gauge deviation of the tube are in an ecentric helical pattern, that among all the operation steps nammely, piercing, Assel pipe mill rolling and stretch reducing, the Assel mill rolling is the most critical step to the W.T. accuracy of the finished pipe and that tubes with different D/S have almost the same absolute W. T. deviation value in quantitive degree. It is concluded that to reduce the absolute value of the tube W.T. deviation is a key approach to enhancing the tube W. T. accuracy.