Well-defined dendrimer-like star block copolymers up to 24 arm were successfully achieved by combination of living ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP) on the basis of poly(aryl ether) dendrimers (CMGn-OH, n = 1-3, with functionality of 6, 12, and 24, respectively). Star polylactides (CMGn-PLLA) were synthesized by bulk polymerization Of L-lactide with multifunctional dendrimer initiators and stannous octoate catalyst. The molecular weights of CMGn-PLLA increased linearly with increasing monomer conversion, with molecular weight distributions remaining below 1.15, indicating the living nature of polymerization. CMGn-PLLA was converted into a macroinitiator CMGn-PLLABr via esterification. with 2-bromoisobutyryl bromide. ATRP of methyl methaerylate (MMA) and styrene (St) using the CMGn-PLLABr/CuBr/pentamethyldiethylenetriamine initiating system afforded star block copolymers with predetermined molecular weights and narrow molecular weight distributions (1.05 < M-w/M-n < 1.20). The molecular weights of star polymers and star block copolymers could be adjusted by the variation of monomer-to-initiator ratios and monomer conversion. The precise arm numbers of 6, 12, and 24 for the resultant star polylactides and star block copolymers were confirmed by NMR spectra analysis and the hydrolysis approach, respectively.