There is an increasing interest in applying back pressure during equal channel angular pressing (ECAP) to improve the process for better control of microstructure and property. The effect of increasing back pressure on deformation characteristics during ECAP such as the plastic deformation zone (PDZ) size and strain rate distribution in the PDZ, size of the corner gap and strain distribution on the longitudinal section were analysed by finite element analysis for both the quasi-perfect plastic and strain hardening materials. This investigation revealed that the back pressure influence very differently the PDZ of the quasi-perfect plastic and strain hardening materials. Many beneficial effects of back pressure were observed in the strain hardening material, with reduced PDZ size, dramatically reduced corner gap, and more uniform strain distribution. For the quasi-perfect plastic material, however, the application of increasing back pressure leads to broadening of PDZ and a decrease in strain rate homogeneity.