A method of trimming hard mask is provided. The method includes providing a substrate, a hard mask layer, and a tri-layer stack on the substrate. The tri-layer stack includes a top photo resist layer, a silicon photo resist layer, and a bottom photo resist layer. The top photo resist layer, the silicon photo resist layer, the bottom photo resist layer, and the hard mask layer are patterned sequentially. A trimming process is performed on the hard mask layer. The bottom photo resist layer of the present invention is thinner and loses some height in the etching process, so the bottom photo resist layer will not collapse.