Low- and high-density polyethylenes were modified using dicumyl peroxide. The modified polyethylenes were subjected to steady and oscillatory shearing flows. The viscous and elastic properties determined with a cone-and-plate rheometer are correlated to the molecular weight of the materials, which was determined with the aid of size exclusion chromatographic analysis. It was found that both the melt viscosity and elasticity increase with the amount of dicumyl peroxide used, and shear-thinning behavior becomes more intense. The Bueche master curve of viscosity is constructed by using a relaxation time based on the weight-average molecular weight for both virgin and modified resins.