The aim of this study was to investigate how changes in the molecular structure of linear low-density polyethylene (LLDPE) during peroxide modification can be detected by a simple rheological method. For this purpose, a commercial-grade LLDPE (Exxon Mobile LL4004EL) was reacted with different doses of dicumyl peroxide (DCP). The samples were analyzed by size exclusion chromatography coupled with a light-scattering detector. With increasing DCP dose, at a roughly constant molar mass, an increasing number of long-chain branches were found. The dynamic shear oscillatory measurements showed a deviation of the phase angle–complex shear modulus curve from that of the linear LLDPE, which was attributed to the presence of long-chain branching. By the use of a simple rheological method that used melt rheology, transformations in the molecular architecture induced on the original LLDPE during the early stages of reactive modification were indicated. Reasonable and consistent estimates of the degree of long-chain branching (x) and the volume fraction of the various molecular species produced in the peroxide modification of LLDPE were obtained. Various three-dimensional plots were constructed to exhibit the correlation between the process parameters and x. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39617.