Abstract: Blade rotation is one of the most important factors thataffect tip clearance flow in turbine rotors. This paper presents a numericalinvestigation of flow structures within tip clearance of the LISA 1.5-stageaxial turbine. It shows that blade rotation obstructs tip clearance flow inaxial turbines. When blades are stationary, relative velocity at gapentrance becomes higher due to the disappearance of blocking effect, toinduce much more fluid into tip clearance. Furthermore, all the leakage flowrolls up to form tip clearance vortex. One more tip clearance vortex nearthe leading edge occurs beside the familiar one at the mid-chord,leading to greater flow losses. Moreover,over-under-turning induced by secondary flow increases with the decrease ofblade rotation speed. Also it can be found that in forward profiles thestatic pressure continues to increase from the entrance to the exit of thetip clearance, and it becomes more uniform in the profile near the trailingedge.