A plastic zone size is regarded as a measure of material resistance, and also it determines the fracture behavior. The importance of estimating or measuring the plastic zone size cannot be too emphasized. In this article, SDSP(Stereo Digital Speckle Photography) is examined as a new techniques to measure the plastic zone developed in a ductile metal. This technique is simple, vibration free and non-contact, and measures simultaneously in-plane displacements as well as out-of plane displacements like the side necking. Experimental results for a CT specimen are presented to demonstrate the accuracy in comparison with those obtained by recrystallization technique and finite element analysis.