olypropylene (PP) accounts for approximately 30% of the total of plastic production in the world. It has been widely used in consumer products (such as automobile, film and packaging materials) and in industrial applications. The amounts of the products are increasing, and also the waste amount is increasing. Many of the waste PP was recycled by thermal energy recovery method. The amount of the material recycle is very few. This is because the physical properties of products made from material-recycled plastics are inferior to those of products made from virgin plastics. Therefore, material-recycled plastics are only used in low-value-added products and have limited applications. However, our recent research indicated that the molecular properties of material-recycled plastics can be restored to the same levels as those seen in virgin plastics. This means that the main factor in the poor physical properties of material-recycled plastics is physical degradation. In this time, we measured the influence of thermal treatment to tensile mechanical property and inner structure quantitatively by using virgin PP to investigate the origin of the physical degradation.