In upset forging, the part is generally formed by a series of operations. Generally, taper upsetting stages are used as preforms. The geometry of the die cavity for the taper preforms can easily be determined from the dimensions of the hot part at the related stages; however the outer dimensions of the header die are determined by using some empirical formulae. In this paper, a commercial finite element code coupled with thermal analysis has been used to simulate hot upset forging process and to examine effects of the process on header die wall and base thicknesses for the taper preform stages. The stress distribution for different wall and base thickness values of the header die has been evaluated and presented. © 2006 Civil-Comp Press.