∵△ABC中，AB=AC，∠A=120°，∴∠ABC= 180°?∠A 2 = 180°?120° 2 =30°，∵△A′BC′由△ABC旋转而成，∴∠CBC′=∠ABC=30°，BC=BC′，∴∠BCC′= 180°?∠CBC′ 2 = 180°?30° 2 =75°．故答案为：75°．
