The melting process of crystalline polypropylene with different sizes was observed on line by means ofdynamic visualization technique through a self- developed assembled vane extruder,and the melting law and mecha-nism of the polymer were also analyzed.The results show that (1) the melting process in the vane extruder includesthree stages,namely the full- solid melting stage,the solid- rich melting stage and the melt- rich meting stage;(2) in the full- solid and the solid- rich melting stages,solid pellets may generate large plastic deformation,and themelting process is dominated by heat conduction and plastic dissipation heat,while in the melt- rich meting stage,themelting process of solid pellets can be obviously observed,and the melting process is dominated by viscous dissipationheat of the melt; and (3) the number of VPCU groups in the melt- rich meting stage is controlled by the hardest melt-ing pellets (namely the largest pellets),and more VPCU groups are required for the melting of small pellets.