1 INTRODUCTION Among various sever plastic deformation methods, equal channel angular pressing (ECAP), which involves sever shear plastic deformation by moving a billet through two intersecting channels, has been the subject of intensive study due to its capability of producing fully dense samples containing ultrafine grain size. In this process, knowledge of the plastic deformation behavior of the work piece during the process is very important for the determination of the optimum process conditions such as die design, speed, temperature, friction and performs design. [1] A generally used equation for calculating, equivalent strain and shear strain experienced by the work piece after one pass of ECAP is given by the following relation: 2 2 csc 2 2 cot 2 3 1 2 2 csc 2 2 cot 2 (1) where and are the die corner angle and the die angle respectively. According to Equations (1), the equivalent strain decrease with the die corner angle . Figure 1 shows the deformation developed according to Eq. (1) using tools with various corner and channel angles. It has been shown that effective strain during ECAP can decrease from a maximum of 1.15 to a minimum of 0.907, while changing the corner angle from 0 to 90 while the channel angle is fixed at 90 . The die channel angle has more influence on the strain generated during ECAP than corner angle. [2]. Extension of the model to other initial element morphologies reveals that the shape change of elements during ECAP is dependent on their initial aspect ratio and orientation in the die entrance channel. Large cumulative plastic strains can be imparted to a billet in multi-pass ECAP processing leading to microstructure refinement and the development of ultra-fine and even nanoscale grains in metals and alloys. ABSTRACT: ECAP due to its capability of producing fully dense samples having ultrafine grain sizes attract much attention. In this process, knowledge of the internal stress, strain and strain rate distribution is fundamental to the determination of the optimum process condition for a given material. Various researchers (Segal, Utyashev et al. Iwahashi et al) calculated total strain experienced by specimens. These results mainly show dependency of the strain on die geometry. The present work focuses on the understanding of the rule of such parameters on strain homogeneity and mean effective strain after one pass. It has been tried to introduce a new equation base on the logarithmic strain. Finally a comparison between theoretical analyses and Iwahashi equation has been carried out that shown accuracy this equation. Key words: Equal-channel angular pressing, Logarithmic strain, Die geometry The Evolution of Strain during Equal Channel Angular Pressing M.H.Parsa, M.Naderi, M. Nili-Ahmadabadi, H. Asadpour School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran- P.O.Box 111155/456, Tehran, Iran e-mail: mhparsa@ut.ac.ir ; mnaderi.eng@gmail.com ; nili@ut.ac.ir; hadi.asadpour@gmail.com