Simultaneous "click chemistry" and atom transfer radical emulsion polymerization (ATREP) to prepare well-defined macromolecules and cross-linked nanoparticles are First reported in this work. First, simultaneous "click chemistry" and ATREP of styrene were conducted in the presence of p-xylylene diazide, using propargyl 2-bromoisobutyrate (PBiB) as initiator, Tween-20 as emulsifier, copper(I) bromideas catalyst, and pent,,pentamethyldiethylenetriamine (PMDETA) as ligand. The simultaneous reactions sharing a same catalyst system gave rise to diblock polystyrene (PS) with controlled molecular weight and narrow molecular weight.distribution. The resulting polymers were characterized by gel permeation chromatography (GPC), FT-IR, and H-1 NMR spectroscopy. Then, well-defined cross-linked PS nanoparticles with diameter in the range of 50-150 nm were prepared from a simultaneous "click chemistry" and ATREP of a mixed styrene and 4-vinylbenzyl azide using 4,4-bis((2'-bromo-2'-methylpropionyloxy)methyl)-1,6-heptadiyne (BMP) as initiator, Tween-20 as emulsifier, copper(I) bromide as catalyst, and PMDETA as ligand. Well-preserved nanostructures of particles in tetrahydrofuran (THF) and dimethylformamide (DMF) solvents reveal the nanoparticles have a cross-linked structure. The size and morphology of nanoparticles were also characterized by photon correlation spectroscopy and transmission electron microscope. The size of nanoparticles is very uniform and can be regulated by changing the ratio of monomer and initiator as well its the amount of emulsifiers. The cross-linked PS nanoparticles were more thermally stabile than PS from ATREP.