Abstract: A series of laboratory hydraulic conductivity (K) tests were conducted to evaluate the potential of two Geo- synthetic clay liners (GCLs) used as a final cover material for coal gangue impoundment. During the experiment,loess leachate was used as permeant liquid. Under a relatively low effective stress of 15 kPa,the K values of both GCLs were approximately 2. 80×10-9 m / s,which were higher than 1. 00×10-9 m / s and couldn’t satisfy the technical requirement for the cover of coal gangue impoundment. However,when the effective stress increased to 90 kPa,the K of the GCL was approximately 3. 52×10-10 m / s,which met the above requirement. The results indicated that there was no distinct difference between the hydraulic performances of the two different GCLs. The results of this study also indicated that increases in effective stress can minimize the detrimental effect of loess leachate permeation on the hydraulic perform- ance of GCLs;however,in order to make the GCL satisfy the requirement,the thickness of the surface loess layer must be higher than 5. 0 m,which is not easy to achieve in the field. Therefore,it is necessary to study GCLs with certain ability of resistance to chemical erosion in future.