An efficient and environmentally benign synthetic method for the production of the stabilized cyclo-olefin copolymer latexes and their carbon nanotube composite nanoparticles has been developed using an emulsion ring opening metathesis copolymerization catalyzed by the 2nd generation Grubbs catalyst in aqueous solution. Homopolymerizations of norbornene (NB) and dicyclopentadiene (DCPD) in aqueous solution yield unstable polymer latexes in combination with a large amount of their flocculation fractions. Copolymerizations of NB or DCPD with a selected liquid cyclo-olefin comonomer dramatically improve not only the colloidal stability of the copolymer latexes but also the thermal stability of the copolymer nanoparticles. The liquid cyclo-olefin comonomer plays a double role as a liquefied agent for the solid NB and DCPD monomers before the emulsification treatment, and a reactive comonomer itself to control entirely the copolymerization system. The as-prepared cyclo-olefin copolymer latexes exhibit an exceptionally high compatibility with a well-dispersed carbon nanotube (CNT) in aqueous solution due to strong - interactions between the graphitic surfaces of the CNT with the C-C double bonds located on the cyclo-olefin copolymer main chains. Accordingly, a binary blending of these two well-dispersed colloidal systems in aqueous solution led to the fabrication, for the first time, of the highly electrical conductive cyclo-olefin copolymer/CNT composite nanoparticles. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4584-4591