Deformation and fracture behavior in aluminum and Al–1.0 mass%Mg alloy wire were investigated under cyclic bending test with the maximum strains up to 0.02 at the specimen surface, in order to develop light weight electric wire for automobiles. Microstructural aspects and geometrical characteristics of the fatigue crack initiation sites were examined where electron back scattering diffraction (EBSD) analysis were employed to make the relationship between partially cracked grain boundaries and activated slip systems clarified. Grain size strongly affected the number of cycles to fracture. The fatigue cracks were generated at the grain boundaries with the inclination angles of 40–60 degrees against the tension-compression direction. Both deformation continuity at the grain boundaries and the work hardening inside crystal grains have effects on the number of cycles to fracture in the cyclic bending.