Miktoarm star polymers were synthesized via atom transfer radical polymerization (ATRP) using the "in-out" method. First, ATRP was applied to the synthesis of star polymers using the "arm-first" method with poly(tert-butyl acrylate) arms and a core formed with divinylbenzene as cross-linker. "Dormant" initiating sites were preserved in the core of the star copolymer, which was then used as a multifunctional macroinitiator (MI) for initiating an ATRP of another monomer to produce the miktoarm star polymer. Because of the congested environment in the star core, not all of the initiating sites participated in the formation of the second generation of arms, and the initiation efficiency (IE) of the star MI was less than 100%. A kinetic method was used for quantitative determination of the IE value of the star MIs. Three factors, including the arm length of star MI, the structural compactness of star MI, and the chemical compatibility of the two kinds of arms on one miktoarm star molecule, have an important effect on the IE of the star MI. It was found that the IE value of the star MI decreased with increasing arm length and structural compactness. When the two arms had the same chemical composition, the star MI had the highest IE value. The IE of the star MI varied from 5% to 54%, depending on the above three factors.