Thermal degradation of NBR and CR heated in air was determined by analysis of diffracted SH ultrasonic waves. Thermal degradation was evaluated by examining wave pattern characteristics of SH waves passing through the surface depth between transmitter and receiver. Thermal crosslinking of main chains in thermally degraded NBR and CR, accompanied by lowering of mechanical properties, can evaluate by decrease of launched time and appearance of 0.9 MHz side peak of the power spectrum. The thermal degradation also causes phase delay at high frequencies, showing regression of viscoelasticity associated with formation of dangling ends.