In order to quantitatively analyze the critical strain for the initiation of dynamic recrystallization in Ni-Fe-based Alloy 718, a series of uniaxial compression tests was conducted in the temperature range 927degreesC - 1066degreesC and the strain rate range 5 x 10(-4) s(-1)- 5 s(-1) with varying, initial grain size. The critical strains were graphically determined based on one parameter approach and microscopically confirmed. The effect of gamma" (matrix-hardening phase) and delta (grain boundary phase) on the critical strain was simply discussed. The constitutive model for the critical strain of Alloy 718 was constructed using the experimental data obtained from the higher strain rate and the temperature range between 940degreesC and 1040degreesC.