In this paper we report Cluster observation of a fast flow event in the plasma sheet associated with a small auroral substorm intensification at 1838 UT on August 12, 2001. Cluster, located in the plasma sheet, experienced significant thinning of the current sheet associated with a high-speed Earthward flow of 900 km/s. By using the four spacecraft magnetic field data and a Harris-type current sheet model, it was estimated that the thickness of the current sheet changes from about 1 RE before the flow observation down to 400 km, i.e., close to the ion inertia length. In the vicinity of this thin current sheet there were also signatures of enhanced current density off the center of the neutral sheet, consistent with recent Geotail results.