In Mg alloys, twins have been known to be an important deformation mechanism. However, their roles on deformation mechanisms have not been well understood. In this work, we performed tensile test of rolled sheets of AZ31 Mg alloys at room temperature. A number of large surface reliefs were observed in a region near a fractured edge. TEM observation showed the formation of twins under the large surface relief. Crystallographic analysis indicated that the basal planes of the twins were tilted by approximately 37° with respect to the basal planes of the matrix, corresponding to the formation of {1011}-{1012} double twins. This is considered to make basal dislocation slip highly active within the twin. The localized deformation within the twins may lead to the formation of crack-like faults.