A method for forming an active pillar of a vertical channel transistor includes forming a hard mask pattern on a substrate, etching vertically the substrate using the hard mask pattern as an etch barrier to form an active pillar, and etching horizontally to remove by-product remaining on the exposed substrate, the hard mask pattern and the active pillar and at the same time to reduce line width of the hard mask pattern and the active pillar, wherein a unit cycle in which the vertical etching and the horizontal etching are each performed subsequently once, respectively, is performed repeatedly at least two times or more. According to the present invention, an active pillar having vertical profiles on its sidewalls and having height and line width (or diameter) required in a highly integrated vertical channel transistor can be provided.