Abstract: Mixed-mode Ⅰ/Ⅱ fracture experiments were performed on aircraft structure aluminum alloy LC4CS. Compact tension (CT) specimens of four kinds of thickness, 2, 4, 8 and 14 mm were used. The coupled effects of thickness and mixed mode Ⅰ/Ⅱ loading on the initial angle of crack are first analyzed systematically, and the applicable scope of the conventional mixed mode fracture criteria is revealed. The experimental and theoretical results are discussed by three-dimensional fracture theory. It is shown that the mixed mode crack has an obvious thickness effect. The maximum tangential stress (MTS-) criterion established on the basis of plane hypothesis is adequate to predict the crack initiation direction of the thin (2mm) and thick (14mm) plates under mixed-mode load conditions. However, it is not suitable to middle-thick plate, especially the 8 mm-thick plates. Although the maximum triaxial stress (M-) criterion considers the influence of three-dimensional constraint on crack initiation, it is not applicable to the aluminum alloy in this paper. Crack initiation angle predicted by the minimum strain energy density (S-) criterion is very close to that by MTS-criterion. It is necessary to develop a universal three-dimensional mixed mode fracture criterion that is valid for various thickness conditions.