The flow structure of a sleeved jet into a main crossflow was experimentally investigated employing particle imaging velocimetry technology and numerically simulated using a CFD code. The jet-to-crossflow velocity ratio, VR, was ranged from 0.5 to 8. Three basic flow patterns were marked, namely attaching jet, lift-off jet and impinging jet as VR gradually increased. The flow in the main duct was characterized by a stream of discharge from the annular space at the rear part of the sleeve near the jet exit, which primarily came from the upstream crossflow. This annulus discharge isolated the leeward wall from the jet fluid and also caused weak local heat transfer in the large momentum deficiency region, and hence could supply an effective protection of the leeward wall from the thermal shock caused by a very cold jet injection. (C) 2003 Elsevier Science Ltd. All rights reserved.