Abstract: The growth and metabolism of fructooligosaccharides( FOS) by Lactobacillus casei in vitro were discussed.The carbon source of 0.5% glucose was replaced by 0.5%,1.5%,3% proportion of FOS.The p H,the number of colonies and the metabolites of the fermented liquid were detected at 0,4,8,12,18,24 h. The results showed that,the p H dropped with the time.The number of Lactobacillus casei in the 1.5% supplementation of FOS reached to the top at 8h.As the concentration increased to 1.5%,the number of Lactobacillus casei increased,but increase the concentration to 3%,there were no significant difference( p > 0.05). The production of acetic acid reached to the top at 4h,and the production in the 3% FOS was 2 times of 0.5% FOS,1.5% FOS.The production of Lactic acid gradually accumulated with the time while 3% FOS was the best,and the main metabolites of Lactobacillus casei were acetic acid and lactic acid.