In this study, we investigated the detwinning phenomenon of in-plane compressed E-form magnesium (Mg) alloy sheet systematically using in situ tensile test combined with electron backscattered diffraction (EBSD) technique. Microstructure and microtexture evolutions were analyzed at different tensile strains during the in situ tensile test. The detwinning phenomenon in E-form Mg alloy was found to be linked to both twin boundary mobility and the interaction of their boundaries with dislocations. The pre-compressed sheet of E-form Mg alloy effectively accommodated the thickness direction strains generated during the in situ tensile test. The effect of detwinning on formability and mechanical behavior of the E-form Mg alloy was also examined. EBSD results indicate that most of the deformation twins formed during in-plane compression were removed when the load is reversed during the in situ tensile test. © 2019, The Minerals, Metals & Materials Society.