Abstract: Certain advantages still stay with the mechanical expanding process for rounding of the large-sized straight welded pipes with rather big sectional ovality. A newly-developed rounding-sizing process for the said pipes is described,which involves pressing the pipe with two half-round dies to produce approximately 0.5% circumferential permanent deformation for the purpose of rounding-sizing of the pipe. The process is verified by means of numerical simulation. The result is worth using as the theoretic basis for R & D of and process preparation for the equipment for rounding-sizing of the large-sized straight welded pipe.