Basal slip and tension twinning are dominant deformation mechanisms of polycrystalline magnesium at low temperature. However, fracture originates mainly from compression twins or shear bands developed from compression twins. This work compared firstly the morphological difference of two types of twins. Then, the dependence of different deformation mechanisms on initial orientations is computed by Schmid factor analysis and compared with measured matrix orientations of twins. Finally, orientation relationships of compression twins with matrices are determined using EBSD technique and compared with theoretical value.