To obtain long branched polybutylene succinate (PBS), modified PBSs were prepared by introducing a branching agent, trimethylol propane (TMP), to the polycondensation system of succinic acid and 1,4-butanediol. Molecular parameters such as absolute molecular weight and root-mean-square (RMS) radius of the modified PBSs were measured by gel permeation chromatography (GPC) with a multiangle laser light-scattering (MALLS) detector. The RMS radius of TMP05, PBS prepared by incorporating 0.5 wt % TMP during polycondensation, was the smallest of all PBSs tested at the same molecular weight. The notable shear thinning and much reduced loss tangent for TMP05 qualitatively supported the difference in their molecular structures predicted by GPC. The analytical results indicated that introducing 0.5 wt % TRIP produced PBS with chain branches long enough to produce molecular entanglements.