Abstract: In order to investigate the effects of the rotor blade tip clearance on the performance of a highly-loaded axial turbine,a single-stage axial transonic scaled-down turbine with low aspect has been tested with four rotor tip clearances.The reduced revolutions of the turbine vary from 80% to 110% and the turbine total pressure ratios vary from 2.0 to 4.0. The experimental results are presented and analysed in the paper.The tip clearance effects on the turbine efficency are predicted with five empirical and semiempirical and the results predicted fomulas are compared with the present experimental results.The experimental results show that the effect of tip clearance on the turbine efficiency is significant,but that on the turbine inlet mass flow is small.When tip clearance increases from 1.054% to 2.823%,the turbine efficiency decreases by 5.0% at the design point,but the mass flow increases only by 0.54%.