Abstract: Abstract: Composition and concentration of volatile flavor compounds of fruit and vegetable juices have important influence on sensory properties and consumer acceptance. The effect of lactic acid bacteria metabolism on the production of bioactive compounds during growth of fruit substrate is well studied, however, the characteristics of the volatile flavor compounds of the fermented juices are less investigated. This paper aims to study the changes of flavor substances and flavor quality of orange juice, apple juice, pear juice, grape juice and cucumber juice before and after fermentation by Lactobacillus plantarum LP-115 400B. Flavor of juices is characterized by interactions between volatile and non-volatile compounds, mainly acids, sugars, and free amino acids (FAA). Quantitative descriptive analysis (QDA) method was adopted to perform the sensory evaluation combed with the analysis of the FAA, sugars and acids. Principal component analysis (PCA) was applied to the sensory evaluation. Volatile flavor compounds were measured by gas chromatography - mass spectrometry (GC-MS) after the preparation of samples by static headspace solid-phase microextraction (HS-SPME). After fermentation, the kind of volatile flavor compounds in different juices increased, which enriched the flavor. The aldehydes were largely reduced to alcohols or oxidized to acids under microbial action. The relative content of alcohols and esters increased significantly, which was beneficial to the apple juice and pear juice, while the wine and pungent smell was increased in the fermented cucumber juice. Plenty of aromatic hydrocarbon was detected in grape juice after fermentation, which enriched the sensory flavor. Because of the changes in terpene, the fresh smell of orange juice decreased and the fermentation odor increased. Due to the metabolism of the lactic acid bacteria during fermentation, the total FAA content significantly reduced, but it still had certain influence on the flavor of fruit juices. The content of sweet amino acids, umami amino acids and bitter amino acids decreased, but the content of aromatic amino acids increased. The changes of FAA in various fruit and vegetable juices showed the correlation between FAA and volatile flavor compounds, such as the increase of the aromatic amino acids and the corresponding increase of the benzene derivatives in different juices after fermentation. The PCA of sensory evaluation showed that the fermented juices were mainly discriminated by the higher intensity of floral, fruity, fermented taste and sour (especially the apple juice and pear juice). The sweet taste of fruit juices was decreased after fermentation, which was related to the decrease of total sugar content and the decrease of sweet amino acids content. After fermentation, pear juice and apple juice showed the most comfortable sweet and sour feeling with the sugars-acids ratio of 21.00 and 17.79 respectively. These findings suggested that apple juice and pear juice may be more suitable substance for lactic acid fermentation to improve the flavor. For the other 3 kinds of juices, reducing the number of initial viable bacteria, terminating the fermentation in advance or adding flavor substances may help to optimize the flavor. In a word, lactic acid fermentation may be considered as an interesting option to enhance the flavor compounds of fruit juices and ensure a better control of flavor changes during juice processing.