A monomer for forming an organic anti-reflective coating layer, a polymer thereof and a composition including the same are disclosed. In a photolithography process, the organic anti-reflective coating layer absorbs an exposed light between a layer to be etched and a photoresist layer, and prevents a photoresist pattern from collapsing due to a standing wave generated under the photoresist layer. The polymer for forming an organic anti-reflective coating layer includes a repeating unit represented by Formula wherein, R1 is a hydrogen atom, a methyl group or an ethyl group, R2 is a C1˜C20 alkylene group, a C3˜C20 cycloalkylene group or a C6˜C20 aromatic hydrocarbon group, POSS is a polyhedral-oligomeric-silsesquioxane, and m is an integer of 2 to 110.