Melt free-radical grafting reactions between EPDM and glycidyl methacrylate (GMA) were investigated in the presence or absence of styrene (St) comonomer in a batch mixer (170 °C, 60 rpm). The effects of dicumyl peroxide (DCP) initiator, GMA, and St concentrations were studied on the grafted EPDM characteristics. Torque–time profiles detected the grafting process by a melt viscosity rise. Titration results indicated an increase in the graft degree (GD) and gel content (GC) values with increasing of the DCP concentration. Introduction of St led to a 132% increase in GD and a 39% decrease in GC by preventing macroradicals from crosslinking side-reaction. Glass-transition temperatures of all samples increased compared with the pure EPDM due to the increase in GC. The increase was greater in the samples containing comonomer because of the stronger intermolecular interactions. Fourier-transform infrared spectroscopy spectra and contact-angle measurements confirmed that GMA was grafted onto EPDM. A calibration curve was developed as a criterion to predict the GD in melt free-radical process.