Plain fatigue and fretting fatigue tests were carried out using cast Al-Si aluminum alloy JIS AC4CH-T6 and forged aluminum alloy JIS 6061-T6. Plain fatigue strength of the cast aluminum alloy was lower than that of the forged alloy. The lower fatigue strength was attributed to casting defects such as pores in the cast aluminum alloy, which enhanced fatigue crack formation. On the other hand, fretting fatigue strength of the cast aluminum alloy almost coincided with that of the forged alloy. No difference of tangential force coefficient and then tangential force in fretting fatigue was observed between the two materials. From fatigue crack growth tests and fretting fatigue tests interrupted at various fatigue cycles, fatigue crack growth curves and fretting crack formation life for both materials were also found to coincide with each other. These phenomena and properties resulted in the agreement of fretting fatigue strengths between the cast and forged aluminum alloys.