A magnetically active, purely organic dot array was formed by the selective deposition of polyradical nanoparticles on array-like-formed pits on a silicon substrate. The nanometer-sized polyradical particles, poly(4-methacryoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl), were prepared by the emulsion polymerization of 4-methacryloy-loxy-2,2,6,6-tetramethylpiperidine-1-acetoxyl followed by a deprotection reaction and oxidation in air. The size (diameter) and radical spin concentration of the polyradical nanoparticles were tunable between the polymerization and oxidation conditions. Electrochemical studies revealed the redox property of the polyradical nanoparticles. The magnetic response image of the polyradical nanoparticles was obtained by magnetic force microscopy, reflecting their radical spin concentrations. These results suggested a possible approach for the use of organic polyradical nanoparticles as organic magnetic dot arrays. (c) 2006 Wiley Periodicals, Inc.