Although low-density polyethylene (LDPE) has been commercially available for decades, a whole picture of its morphology related to micro-structure has not been yet clarified. Here, several characterization techniques, i.e., C-13-NMR, triple-detector gel permeable chromatography (3D-GPC) and rheology, are integrated to depict its detailed structure. The results yield a new proposed concept of primitive core structure existence prior to an addition of long-chain branch (LCB), the so-called "Primitive Core" (PCore). Morphology of the PCore can be estimated from the correlation plot of the related-LCB parameters obtained by C-13-NMR and 3D-GPC, i.e., LCBcor-NMR and gpcBR, respectively. It is not only the detailed morphology of the core that is revealed, but also the morphology of LCB growth is proposed. The correlation plot of the LCBcor-NMR and the LCB index (LCBirheo calculated from rheological information) shows two discrete LCB-growth regimes: the LCBs grow onto (I) the PCore (or, branch on PCore, BoP) and (II) on the exiting branches (or, branch on branch, BoB). Furthermore, from those parameters, the number and the average length of LCB can be estimated.