The fatigue processes of neat polymers, alloys and composites have been investigated by measuring mechanical work, heat generation and percent of work that appears as heat (PWAH) under cyclically mechanical load on the specimens. PWAH is considered to serve as an index by which a fatigue life can be predicted. The larger is PWAH, the longer is the fatigue life of the polymers. PWAH experimentally depended on the entanglement density of a polymer. This means that the polymer chains entangle with one another and the heat generated by friction of the polymer chains does not lead to deformation of the polymer matrix. In the case of alloys and composites, the fatigue life depended on the structure of the interface between different phases and especially on the entanglement and chemical bonding in the interfacial region, PWAH was effective in predicting the fatigue life and analyzing of the mechanism.