Abstract: In this study, effects of acid stress on membrane characteristics were investigated by using Lactobacillus casei as initial strain. The results showed that lateral diffusion coefficient decreased and the micro- viscosity increased with the decrease of pH.In addition, analysis of the inner membrane permeability and concentrations of intracellular metal ions showed that acid stress led to increased permeability of cell membrane.The distribution of membrane fatty acids was investigated, and the results showed acid stress induced increased unsaturated fatty acids and decreased saturated fatty acids. Moreover, acid stress resulted in increased unsaturation degree and chain length of membrane fatty acids. Results presented in this study provided theoretical basis for further elucidation of the acid tolerance mechanisms employed by lactic acid bacteria.