Abstract: Bentonite-modified loess has been used as bottom liner material for municipal waste landfills. However, because the diffusion and adsorption behavior of pollutants was not determined, the performance of the bentonite-modified losses could not evaluated. Two bentonite-modified loess liners were tested in the present study, containing 15% bentonite and 30% (by weight) bentonite, respectively. A series of batch adsorption and diffusion tests were conducted to examine the adsorption and diffusion behavior of ammonium in bentonite-modified loess liner. The test results show that the addition of 15% bentonite could not increase the adsorption capacity of ammonium on the modified liner, whereas it could decrease the diffusion coefficients of ammonium in the modified loess liner by about 400%. When 30% bentonite was added, the adsorption capacity of ammonium on the modified liner increased by a factor of 1.25, whereas, the diffusion coefficient dropped by about 420%. Pollute v7.0 model results show that it would produce 3.50～4.75 times errors if the diffusion behavior of pollutants were not taken into account when estimating the performance of the bentonite-modified liner. Whereas, there would be 334～382 times errors in the estimating of the modified liner performance if the adsorption behavior of pollutant was not considered. The results of the present study provide parameters for the evaluation of bentonite-modified loess liner performance.