Upsetting tests were conducted on cylindrical specimens made of a typical fresh short fiber-reinforced cementitious composite (SFRCC) paste for extrusion at various boundary conditions to derive the paste's constitutive behavior. Due to friction present at the specimen and the equipment platens interfaces, stress was not uniformly distributed in the specimen and had to be corrected. In this study, two analytical methods were utilized to derive the true flow stress of the fresh SFRCC paste through correcting the nominal yield stress obtained from the upsetting test. One of them was the stress extrapolation method, which required a large number of experimental data and predetermined boundary friction. It was found that the results corrected by this method from experiments at the lubricated boundary were more reliable than those at the dry boundary and this method could also give an approximate estimation of the boundary friction coefficient. As comparison, the flow stress correction method required only a small amount of experimental data and no predetermined boundary friction. It was found that this method provided better results at the dry boundary than at the lubricated boundary. The fresh SFRCC paste exhibited both strain and strain rate hardening characteristics whereas strain rate hardening effect was more significant and dominated. © 2006 ASCE.