In-situ observation of small fatigue crack growth in α-brass was performed by means of an atomic force microscope (AFM) equipped with a small three-point bending fatigue testing machine. The fatigue crack tip could be located exactly by measuring the variation of crack opening displacement (COD) during one load cycle. It appeared that there was the dependency of the frequent branching and deflection of the main crack on the slip deformation observed on the specimen surface.