Precise morphology control of ultrathin films is one of the important issues in nanotechnology. To this end, we describe that various controlled morphologies of hierarchical phase separation can be attained using poly(methyl methacrylate)-b-poly(n-butyl acrylate) (PMMA-b-PBA) monolayers spread on a water surface. At a low surface pressure, they were miscible, but upon compression, phase separated with a monolayer of the major component block spreading on the water surface, on top of which the minor component block separated out (hierarchical phase separation). The morphology of the minor component block separated out on top of the monolayer varied from a sphere short string mixture, to long strings, and a mesh-like structure having a regular domain-domain spacing with the increasing content of the minor block, and the hierarchical phase separation was reversible depending on the surface pressure. However, these well-ordered hierarchical phase separations were observed only in the PBA-rich polymers, and a clear morphology was not observed in the PMMA-rich polymers, because of insufficient domain growth after the hierarchical phase separation for the PMMA-rich block copolymers which are more rigid than the PBA-rich polymers. Although the clear hierarchical phase separation was limited in flexible composition, we noted that the hierarchical phase separation provided a unique well controlled morphology in the Langmuir monolayers. (C) 2016 Elsevier Inc. All rights reserved.