The dominant deformation mode at low temperatures for magnesium and its alloys is generally regarded to be twinning because of the hcp crystal structure. More recently, the phenomenon of a “loss” of the twins has been reported in microcompression experiments of the magnesium single crystals. Molecular dynamics simulation of compression deformation shows that the pyramidal (a + c) slip dominates compression behavior at the nanoscale. No compression twins are observed at different temperatures at different loadings and boundary  conditions. This is explained by the analyses, that is, the {1012} and {1011} twins can be activated under  c-axis tension, while compression twins will not occur when the c/a ratio of the hcp metal is below 3.Our theoretical and simulation results are consistent with recent microcosmpression experiments of the magnesium (0001) single crystals.