In the present article, an elongation viscosity equation of polymeric melts was derived based on the Moore dynamic model. The equation described the relationship between extensional viscosity and extension strain rate of polymer melts. The effects of the four parameters in this equation on the extension viscosity curves were analyzed in detail. The extensional viscosities of a low-density polyethylene (LDPE) and a linear LDPE as well as LDPE/glass bead composite melts at 170°C were estimated by applying this equation, and the predictions were compared with the measurement data under the same experimental conditions. The results showed that the estimations were in good agreement with the experimental data.