The characteristics of poly(methyl methacrylate) (PMMA) etching of self-assembled poly(styrene-block-methyl methacrylate) (PS-b-PMMA) thin film for forming a polystyrene (PS) mask were investigated. In this investigation, first, the etching selectivity of PMMA to PS under argon- and oxygen-plasma processes was evaluated. Higher selectivity was obtained in the case of argon plasma (3.9) compared to that of oxygen plasma (1.7). Second, to investigate the argon process in detail, the time dependence of etching depth was evaluated. It was found that PMMA etching rate decreases by more than half after etching to a depth of around 15 nm. To investigate the mechanism of this decrease in PMMA etching rate, the surface composition of PMMA was measured by X-ray photoelectron spectroscopy (XPS). The XPS result revealed that the reduction of etching rate is caused by a depletion of oxygen by argon ions, and the depleted oxygen attaches to the PMMA film in air exposure. In accordance with these results, to compensate the decrease in oxygen concentration, oxygen was added to the argon plasma at a composition of 1%. As a result of this oxygen addition, constant PMMA etching rate was confirmed, even beyond etching depth of 50 nm. It is thus concluded from these results that a PS lamellar mask pattern with a pitch from 25.5 to 70 nm could be successfully formed by using selective PMMA etching.