In this study, stress corrosion cracking (SCC) behavior of AZ31 magnesium alloy was carried out using slow strain rate testing (SSRT) technique in 3.5 wt% NaCl solution. The influence of microstructural scale on the stress corrosion behavior was investigated in AZ31 alloy with three different mean grain sizes. Single-pass and two-pass friction stir processing (FSP) was employed to obtain fine grain and ultrafine grain microstructures, respectively. For FSP, SSRT specimens were extracted from the processed region. SSRTs were carried out in air and solution at an initial strain rate of 10(-6)/s. A significant decrease in the ultimate tensile strength was observed for FSP specimens tested in chloride solution as compared to specimens tested in air. More than 75 % loss in total elongation was observed for the specimens tested in chloride solution as compared to the ones tested in air. In comparison with base material, lower time to failure was observed for processed samples. The higher SCC susceptibility of processed microstructure is attributed to increased hydrogen adsorption and favorable basal texture.