Both the KS1275^® piston and AE42 alloys and their composites have realistic and/or potential applications as engine components in the automotive industry. Used as engine components, the dimensional stability is of great concern. Thermal cycling experiments can simulate the service conditions of the materials and give an evaluation how the dimension changes during their service in the changing temperature environments. The present paper investigates the thermal fatigue of the short fiber reinforced KS1275^® piston and AE42 alloys, with an emphasis on the changes in the strain and hardness before and after thermal cycling. The effects of fiber orientation and composition, and subsequent heat treatment, on the thermal strain were discussed. It is shown that the thermal strain was affected by experimental condition of the thermal cycling and the strength of matrix. After thermal cycling, the hardness decreases due to the occurrence of the matrix overageing and recovery.