Abstract: Reinjection into landfill is one of common methods for treating landfill leachate membrane concentrate. However, the effect of landfill leachate membrane concentrate (LLMC) permeation on the hydraulic performance of geosynthetic clay liners (GCLs) remain unclear until now. In the present study, a series of hydraulic conductivity tests were conducted to investigate the behavior of the hydraulic conductivity of GCLs permeated with two different LLMCs (i.e., typical and strong LLMC). Free swell index tests and scanning electron microscope tests were carried out to reveal the possible mechanism responsible for the changes in hydraulic conductivity. The test results show that both the permeation of LLMCs could result in about 1.96 to 54.72 factors increase in the hydraulic for the GCLs. For a certain GCL, the impact of strong LLMC was more pronounced than that of typical LLMC. The sodium-activated bentonite GCL was more sensitive compared to the natural sodium bentonite GCLs when permeated with the same LLMC. The increase in the hydraulic conductivity could be mainly ascribed to the contraction of the electric double layer of the bentonite particles. All the results show that the LLMC could decrease the hydraulic performance of GCL more or less. Thus, the recirculation of LLMC should be limited in the practical field. This study provides a theoretical foundation for the assessment of hydraulic performance of GCLs used in municipal solid waste landfill.