Mg alloy has hexagonal structure and exhibits poor workability at room temperature, which is attributed to the difficulty in activating a sufficient number of independent slips to accommodate the deformation. Twinning plays an important role in plastic deformation of Mg alloys during low and medium temperature to accommodate the imposed strain, especially the strain along the c-axis. Therefore, the twinning behavior of AZ31 Mg alloy during plane strain compression at room temperature was investigated with EBSD in this work. Rectangular specimens with a dimension of 10 mm in length, 9 mm in width and 7 mm in thickness were cut from a hot rolled plate. The results show that {101-2} twinning is dominant when the compression and constraint direction are parallel to transverse direction (TD) and rolling direction (RD) of the plate, respectively. The twinning variant selection mechanism is dominated by the Schmid factor (m) along compression direction, and also related to the constraint direction. The differences of twinning behavior can be interpreted by the twinning strain tensor. For the case when single twinning variant occurs within a grain, the average twinning strain tensor of twinning variant in constraint direction will result in spreading; while for the case there are two or more twinning variants taking place within a grain, the average twinning strain tensor of the variant with higher m will induce spreading in the constraint direction, and that with lower m results in size reducing in the constraint direction. During plane strain compression, different twinning variants coordinate with each other, twinning won't be suppressed until the micro-strain in the constraint direction reaches 0.