We report the synthesis of stable nanometer‐size silver‐coated polydiacetylene (PDA) particles. At first, an ethanol solution of diacetylene (DA) is dispersed into a Ag+ion containing aqueous solution. Silver ions are attracted to –CCO−anions of DA, then monomer DA vesicles coated by silver ions are formed by a long‐term ultrasonication treatment. The monomer DA vesicles are photopolymerized and the silver ions are photo‐reduced simultaneously to form coated composite particles. ©1996 American Institute of Physics.