A novel silicone‐based positive photoresist (SPP) is developed for a two‐layer resist system. SPP is composed of an alkali‐soluble silicone polymer (APSQ) and a diazonaphthoquinone compound as a sensitizer. APSQ is newly synthesized by means of acetylation of polyphenylsilsesquioxane (PSQ). A fine SPP pattern below 0.5 μm can be fabricated with a g‐line stepper (NA=0.6) using a two‐layer resist system. Moreover, a 0.4 μm line/space pattern is successfully fabricated at a defocus of ±0.4 μm. The high oxygen reactive ion etching (O2RIE) resistance of SPP makes possible the fabrication of a submicron pattern with a high aspect ratio in a two‐layer system. Pattern width loss can be negligible during O2RIE of a 2 μm thick bottom resist when a 0.6 μm thick SPP is used as the top imaging layer.