This article describes the formation and characterization of self-assembled nanoparticles of controlled sizes based on amphiphilic block copolymers synthesized by ring-opening metathesis polymerization. We synthesized a novel hydrophobic derivative of norbornene; this monomer could be polymerized using Grubbs' catalyst [Cl2Ru(CHPh)(PCy3)(2)] forming polymers of controlled molecular weight. We synthesized amphiphilic block copolymers of controlled composition and showed that they assemble into nanoparticles of controlled size. The nanoparticles were characterized using dynamic light scattering and transmission electron microscopy. Tuning the composition of the block copolymer enables the tuning of the diameters of the nanoparticles in the 30- to 80-nm range. (C) 2004 Wiley Periodicals, Inc.