Star poly(methyl methacrylate)s (P*) of various arm lengths and core sizes were synthesized in high yields by the polymer linking reaction in Ru(II)-catalyzed living radical polymerization. The yields of the star polymers were strongly dependent on the reaction conditions and increased under the following conditions: (1) at a higher overall concentration of arm chains ([P*]), (2) with a larger degree of polymerization (DP) of the arm chains (arm length), and (3) with a larger ratio (r) of linking agents to P* (core size). In particular, the yields sharply increased in a short time at a higher temperature, in a polar solution, and at a higher complex concentration after the addition of linking agents. These star polymers were then analyzed by multi-angle laser light scattering to determine the weight-average molecular weight (3.8 X 10(3) to 1.5 X 10(6)), the number of arm chains per molecule (f = 4-63), and the radius of gyration (R-z = 2-22 nm), which also depended on the reaction conditions (e.g., f and R. increased as [P*], DP, and r increased). Small-angle X-ray scattering analyses of the star polymers showed that they consisted of spheres for which the radius of the microgel core was 2.7 nm.