Reactive processing on conventional poly(ethylene terephthalate) (PET) with the combination of pyromellitic dianhydride (PMDA) and triglycidyl isocyanurate (TGIC) was used to produce long-chain branched PET (LCB-PET). The branching factors of the PET samples were correlated from their intrinsic viscosity and molar mass results. In all PET samples without a cross-linked structure, PET modified with 0.1 wt % PMDA and 0.5 wt % TGIC (PET-P1T5) had the highest branching degree, with a branching factor of 0.84. The relaxation time spectra of the three modified PET samples with strain-hardening behaviors displayed rubbery states during the relaxation process, which was ascribed to their increased molecular weight or long-chain branched structure. Batch melt-foaming experiments with CO2 were carried out to evaluate the melt-foamability of PET. The results indicated that both a high molecular weight and long-chain branched structure, especially the latter, could efficiently lead to strong molecular entanglements and significantly improve the polymer melt-foamability.