Recently, the fuel consumption improvement by weight saving is required in transportation machine. Therefore, Mg alloy is expected as the alternative material of Al alloy and resin material, because it is the lightest in the practical metals and its specific intensity is high. However, twin crystal deformation occurs easily, and there are the two kinds types (i.e. {1012} and {1011} twin crystal) in that deformation. It has been reported that the {1012} twin crystal has work to relax the stress concentration and the {1011} twin crystal has close relation to fracture. Our aim is to evaluate the kinds and the conditions of those types with AE method. In this study, Mg alloy AZ31 was used, and the AE signals of the alloy were detected in compression, tensile and CT test. After that, the analysis of those signals were done. From the results obtained under this experimental conditions, the following things became clear. (1) The frequency components near 250, 550 and 850 kHz are caused by slip, twin crystal deformation and crack growth respectively. (2) It is possible that the two kinds crystal deformation types are discriminated by the identification standards decided by the AE count and fractal dimension Fd of the detected signals.