Abstract: Abstract: Most bacteria in the natural environment choose to live in the biofilm state, due mainly to better advantage over the planktonic state. The resistance of strains to the external environment can be significantly improved, when lactic acid bacteria behave in the form of a biofilm state. Therefore, it is a benefit to investigate the stress resistance of strains in biofilm state for the production mechanism behind the lactic acid bacteria biofilm under environmental stress. In this study, two strains of Pediococcus acidilactici RJ2-1-4, TG1-1-10 and two strains of Lactobacillus plantarum RJ1-1-4, RM1-1-11 (They were both high-yield biofilm strains) were selected to systematically explore the tolerance of planktonic and biofilm strains to acid, alkali, bile salt, simulated artificial gastrointestinal fluid, and antioxidant ability. The results showed that the growth of strain was inhibited under the condition of extremely acid, but the growth of biofilm state RM1-1-11 at pH 3.0 was significantly higher than that of the planktonic state (P<0.05). The density of bacteria increased with the increase of pH value, whereas, the alkaline environment in pH 7.0-9.0 inhibited the growth of three strains except TG1-1-10. The growths of membranous strains RJ2-1-4 and TG1-1-10 were significantly lower than those of planktonic state (P<0.05), particularly that the growth of strain increased slightly, when the concentration of bile salt was 0-0.03%. However, the growth of strain was inhibited as the concentration of bile salt continued to increase. In addition, the TG1-1-10 growth of planktonic strain was higher than that of biofilm strain, whereas the growth of the other three strains in the biofilm state was significantly higher than those of planktonic strain. After the strains were treated in the simulated artificial gastrointestinal fluid for 3 hours, it was found that the survival rate of biofilm strains in the gastric and intestinal juices improved, compared with the planktonic strains. There was a certain clearance ability of four strains for different kinds of free radicals. The clearance rates were ranked in order: HO·>DPPH·> lipid peroxidation > superoxide anion. Specifically, the clearance rate of RJ2-1-4 planktonic bacteria suspension on DPPH· was 214.12 μg/mL, while the clearance rate of RJ2-1-4 biofilm CFS to superoxide anion was 93.8 μg/mL, and the clearance rates of RJ2-1-4 biofilm CFS and TG1-1-10 planktonic CFS on HO· were 713.81 μg/mL and 637.01 μg/mL, respectively. The clearance rate of lipid peroxides by RM1-1-11 biofilm suspension was 122.82 μg/mL. Lactic acid bacteria in biofilm state had certain protective effects on acid, alkali, bile salt and artificial gastrointestinal fluid, but there was specificity among strains, even in the same genus. The anti-oxidation ability of biofilm strain was higher than that of planktonic state, but there was a definite difference in different kinds of free radicals. The finding can provide a significant support to a further investigation on the resistance of lactic acid bacteria to environmental stress in biofilm states.