Abstract: Briefed here in the essay are the purpose and advantages of the modifying a small-sized horizontal barrel-roll piercing mill into a cone type piercing mill with a 5° toe angle.Also elaborated are the difficulties as encounter with during the modification without major change of the mill proper. Details involve the separate re-design and re-verification of the roll type, the roll bearing seat, the bearing box, and the driving spindle., and particularly, the issue whether the strength of the driving spindle and the half flange as changed with their dimensions can still meet the process requirements, and the possible suitability of the maximum and the minimum roll diameters for the product size class interval. The practice shows that thanks to relevant theoretic calculations and detailed designs, the one-time success of the trial rolling operation of the said modified piercing mill is realized.