The microcrack tip plastic zone sizes in austenitic steels are measured using REM and interference microscope. It is shown that the plastic zone size varies from 300µm to 350µm. The importance of determining this parameter is discussed. Based on the analysis of the conventional continuum equations of linear-elastic approach a simple formula is derived for calculation of plastic zone size, R=d E/2π σF, establishing relation between the plastic zone radius (R), microcrack width (d), elasticity modulus (E) and the yield strength of the material (σF). The measured values of plastic zone size are in a good agreement with those reported in literature, and calculated by the above formula.