A novel hyperbranched poly(phenylene oxide) (HPPO) with phenolic terminal groups was prepared from 4-bromo-4′,4″-dihydroxytriphenylmethane as AB2 monomer in dimethylsulfoxide (DMSO) via a modified Ullmann reaction.The molecular weight and polydispersity (PD) of the resulting polymers increased with increasing reaction time. In the presence of core molecules (bisphenol A and 1,3,5-trihydroxybenzene), which have the similar molecular backbones to the reactive monomer, the molecular weight could be controlled by varying the core-to-monomer ratio. Incorporation of a very small amount of core molecules could lead to a higher molecular weight as compared with that without the addition of core molecules. However, when the core content reached certain extent, the molecular weight would decrease with the further increase in the core content. A new similar behavior of control over the PD was also obtained. The resulting polymers were characterized by 1H-NMR, 13C-NMR, FT-IR, and GPC.