A new conjugated polymer, poly[{2-methoxy-5-(2-ethylhexyloxy)-1,4-(1-cyanovinylphenylene)}-alt-co-{2,5-bis(N,N'-diphenylamino)-1,4-phenylene}] (DB-CN-PPV), with strong UV absorption and high fluorescence in red wavelength region, is self-assembled into nanoparticles (polymer dots, Pdots) in aqueous solution upon rapid mixing of the organic solution with water. Different-sized Pdots were obtained by varying the organic solvent percent in the mixture solution and further characterized by dynamic light scattering and transmission electron microscopy. Spectroscopic measurements indicate that the DB-CN-PPV Pdots exhibit a bright red-emitting and a quantum yield of similar to 27 %. Single-particle imaging and analyses indicate the brightness of DB-CN-PPV Pdots under 405 nm excitation is four times brighter than that of the well-known inorganic quantum dot probes, Qdot655, under same excitation light. The successful bioconjugation and cellular imaging suggest DB-CN-PPV Pdots have potential application for biological fluorescence imaging.