The surface tension of Colloidal Unimolecular Polymer (CUP) suspensions at the air/ water interface was measured as a function of concentration and molecular weight. the surface activities of the CUP particles were found to relate with the molecular weight of polymer. These nano scale particles, 3-7 nm in diameter, follow the size distribution of the GPC absolute molecular weight. the charge density of these particles was based upon the acid content of the resin. the relationship between size and surface tension was found to be different from that of latex, polyelectrolytes and silica particles. It may originate from the high surface charge density of the particles and their small size.