The turbulence statistics of an unsteady separated ยฏow was experimentally investigated. A backward-facing step ยฏow at Re 3700 was chosen as the test case, where periodic perturbation was introduced from its step edge. A two-dimensional particle imaging velocimeter (PIV) was used in the ยฏow-ยฎeld measurement. The measured results showed that the reattachment length was reduced by the applied periodic perturbation. There existed an optimum frequency for the promotion of the reattachment. When perturbed at the optimum frequency, St 0:19, the reattachment length was reduced by 30%. The Reynolds stress components were increased by the perturbation, and their distribution varied with the perturbation frequency. When perturbation at the optimum frequency was applied, Reynolds stress markedly increased near the reattachment zone. This increase in Reynolds stress enhanced the momentum transfer across the shear layer, enabling the promotion of the reattachment. On the other hand, the region where the perturbation at higher frequency than the optimum frequency increases Reynolds stress was limited immediately behind the step. The perturbation at lower frequency than the optimum frequency increased Reynolds stress in the region downstream of the reattachment zone. Therefore, both low-and high-frequency perturbations have less eect on the promotion of the reattachment than optimum-frequency perturbation.