Abstract: All the stress components at a rapidly propagating crack-tip in elastic perfectly-plastic material are the functions of only. Making use of this condition and the equations of steady-state motion, plastic stress-strain relations, and Mises yield condition with Poisson ratio, in this paper, we derive the general expression of perfectly plastic field at a rapidly propagating plane-strain crack-tip. Applying this general expression with Poisson ratio to Mode I crack, the perfectly plastic field at the rapidly propagating tip of Mode I plane-strain crack is obtained. This perfectly plastic field contains a Poisson ratio, and thus, we can obtain the effect of Poisson ratio on the perfectly plastic field at the rapidly propagating tip of Mode I plane-strain crack.