Abstract: Set up in 1960 and located at the Jiangyin Municipal Seamless Steel Tube General Plant, the first China-made Φ100 mm plug pipe mill had been used for more than four decades. Although it had been since modified for several times at its original site, it was still rather backward in terms of technological configuration, and failed to produce quality tubes as required by the market. The paper focuses on the major technical upgrade of the said plug mill line performed after it is relocated, covering such advanced technological processes as heating billet with the rotary hearth furnace, billet hot-centering, mid-frequency induction furnace reheating and slight stretch sizing (reducing), etc. As a result, the mill's product size range is expanded, product quality improved considerably and the competitiveness enhanced.