Patterned films of a low-polydispersity polymer densely end-grafted on a silicon substrate were fabricated for the first time by the combined use of the Langmuir-Blodgett (LB) and the surface-initiated atom transfer radical polymerization (ATRP) techniques: a blond monolayer of 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CTS: ATRP initiator) and n-octadecyltrimethoxysilane (OTS: non-initiator) was immobilized on a silicon wafer by the LB technique, and then tile ATRP or methyl methaerylate was carried out on the modified wafer in the presence of the Cu/ligand complexes. Atomic force microscopic studies revealed that the CTS/OTS blend was immiscible and phase-separated into two monolayer phases: most OTS molecules aggregate with each other, forming a condensed-type monolayer domain with CTS molecules excluded from there almost perfectly, and the remaining OTS molecules are incorporated in the matrix region. This 2-dimensionally phase-separated structure was successfully amplified by the controlled growth of a high-density graft layer only on the matrix phase of CTS as a main component. The amplification by the ATRP technique was characterized by a sharp boundary between the grafted and ungrafted domains; as a measure of the spatial resolution, the boundary sharpness Deltaw was evaluated to be ca. 100 nm. The domain size in the phase-separated structure was independent of the mole fraction of CTS, while it could be changed by changing the pH or the subphase water: namely, the higher was the pH, the larger was the domain size. It was deduced that a change in pH of the subphase water gave rise to a change in the hydrolysis rate of the methoxysilyl groups into silanol groups and hence a change in the rate of condensation of the silanol groups into domains.