解:∵四边形ABCD是矩形,∴AC=BD,OA=OC,OB=OD,∴OA=OB,∵∠AOB=60°,∴△AOB是等边三角形,∴OA=OB=AB=2cm,∴AC=2OA=4cm.故答案为:4.
∵四边形ABCD是矩形,∴AC=BD,OA=OC,OB=OD,∴OA=OB,∵∠AOB=60°,∴△AOB是等边三角形,∴OA=OB=AB=2cm,∴AC=2OA=4cm.故答案为:4.