The cord fatigue in running tires is brought about mainly by heat, cyclic elongation and compression. These individual effects have been reported in previous papers. Here, reciprocal actions of heat, cyclic elongation and compression on tire cord are studied in order to make clear the fatigue mechanism of tire cord in real tires. The results obtained are as follows:(1) The changes of fine structure in tire cord caused by compression are accelarated by reciprocal actions of elongation and compression, while the changes caused by elongation are not. This phenomena are explained by the increase of compression stresses corresponding to the amount of plastic flow caused by elongation. Hence, inter-planar slip in crystallite in tire cord is promoted and filament strength decreased earlier to break down the cord when tire cord is fatigued by cyclic elongation and compression.(2) Thermal effects are enhanced on the fatigue phenomena of tire cord caused by cyclic elongation and compression, such as break-down or flow of molecular chain in amorphous region and inter-planar slip in crystallite, in addition to thermal degradation.