With the visualization corotating twin-screw extruder, the melting of HDPE pellets under different screw configurations and operational conditions was investigated on-line. It is shown that the melting progression of polymer pellets in corotating twin-screw extruders is different from that in single-screw extruders. Screw configuration, screw speed, and feed rate are three decisive factors in polymer melting. Based on the melting phenomena, a new melting mechanism, island-sea melting model, was defined. It was found that island-sea melting model is an important mechanism to realize high efficiency melting progression. The state of polymer, the degree of fill, and the structure of kneading blocks are factors in determining the energy consumption of kneading blocks in melting zone. The conveying of polymer pellets in kneading blocks, in which pellets were compressed and deformed, consumed much more energy than the conveying of polymer melt. This indicates that the control of the deformation of polymer pellets in corotating twin-screw extrusion is a useful concept in developing new screw elements for high efficiency polymer melting.