Branched polystyrene was synthesized through controlled radical polymerization of styrene in the presence of 4-(4'-vinylphenylmethoxy)-2,2,6,6- tetramethyl-1-piperidinyloxy (STEMPO) which possesses a nitroxyl stable radical and a polymerizable vinyl group. During the polymerization, STEMPO played the role of branching agent since it can polymerize as well as trap the propagating radical. The resulting polymer contained a labile covalent C-ON bond at each branch point and had broad molecular weight distribution. After being heated with phenylhydrazine, the GPC profile of the product became narrower due to cleavage of the branch points. The molecular weight of the cleaved product was proportional to monomer conversion, demonstrating a controlled growth of primary chains. H-1 NMR showed that, in the polymerization of deuterated styrene, the signal intensity of vinyl groups in STEMPO decreased at a constant rate with increasing monomer conversion. ESR measurement illustrated that the labile C-ON bonds at branch points were subject to thermal homolysis. The branchcd product was used as a macroinitiator for radical polymerization of 4-methoxystyrene. The resulting copolymer possessed a branched architecture of diblock primary chains with controlled length and narrow distribution.