The distributions of the number of arms (DNA) of (polystyrene)(6.2)-star-polyisoprene ((PSt)(6.2)-star-PIs) and (PSt)(7.6)-star-(PIs)(6.3) hetero-star block copolymers and its precursors prepared via anionic living polymerization of macromonomers have been determined by plotting a weight fraction of the number of arms against the number of arms. Three-dimensional molecular weight distributions (3D-MwD) of the two hetero-star block copolymers and its precursors were also determined by plotting a weight fraction at the ith elution volume against the corresponding molecular weight (M-i) and ith elution volume using GPC in conjunction with a low-angle laser light-scattering detector, a refractive index detector, and an ultraviolet detector. A non-linear relationship between the ith elution volume and M-i of (PSt)(6.2)-star-PIs was observed and explained from a g(star) perspective. The present paper describes the analytical method for determination of the DNA using the MwD. The DNAs of the six samples were found to be narrow and similar to the M(w)Ds of the corresponding samples. The synthetic mechanism of the hetero-star block copolymers was discussed from a DNA perspective. (C) 2009 Elsevier Ltd. All rights reserved.