Bending tests were performed for a micro-sized cantilever specimen that made at Fe-2wt%Sn alloy by using focused ion beam and the root of the specimen was set to a grain boundary to investigate strength for grain boundary fracture by using an atomic force microscope with ultramicro hardness tester. The alloy was heated at 600℃ for 5 h in argon gas atmosphere to cause grain boundary segregation of Sn. Three specimens were prepared for a random grain boundaiy and each specimen had a notch with different depth from 0.7μm to 2.3μm on the grain boundary. The three specimens were fractured at the grain boundary in the bending tests. The J integrals evaluated around the notch tips of three specimens by finite element analysis were between 158 J/m^2 and 171 J/m^2. The J integrals were equal within 5% independent the notch depth, indicating that the quantitative evaluation for the strength of grain boundaries as J integral can be attained by adopting this experimental method.