A micro tensile testing method for evaluating grain boundary strength was developed. Specimens of 2×2×10 μm having one grain boundary were made by focused ion beam (FIB) micro-processing and tensioned in an FIB system. The load was measured from the deflection of the Si cantilever and the displacement was also monitored in situ. The method was applied to aged and unaged Fe-Mn-P alloy specimens with different grain boundary P segregation. The load at intergranular fracture decreased with increasing P segregation.