In order to investigate {1012} tensile twinning behavior, the magnesium single crystal was deformed by compressing along the [21 10] direction at room temperature, as {1012} tensile twinning easily takes place when the compression direction is perpendicular to the c-axis. Numerous {1012} primary tensile twins were activated during deformation, and the Schmid factor (SF) criterion was applied to the six {1012}twin variants. The analysis shows that the majority of the {1012} primary twins belong to high SF variants,and high SF twin boundaries provided nucleation sites for low SF variants. The {1012} secondary tensile twins were formed inside the high SF of wide {1012} primary twin bands, and the basal plane of the {1012} secondary twin was tilted about 60° with respect to the original parent matrix. In the case of the {1012} secondary tensile twin, relatively low SF variants were activated while counterparts with higher SF variants were absent.