This experiments have objective to study factors affecting fermentation and flavor of fermented leaf mustard (Brassica juncae L.). Leaf mustard were used as raw material which had moisture content 94.9%, total sugars (as invert sugar) 0.031%, total acidity (as citric acid) 0.281%, pH 5.86, total plate count 5.03x10x10x10x10x10x10 colony/g and lactic acid bacteria 1.86x10x10x10x10x10x10 colonylg. Salt and sugar concentration, temperature and microorganism are the main effect controlling fermentation. Two levels of salt concentration in brine (4 and 10%w/v) were studied. The results showed that pH and total sugars decreased in 4% salt more than in 10% salt (p<=0.05). But %total acidity, total plate count (TPC) and lactic acid bacteria (LAB) increased (p<=0.05). The results of sugar concentration (1 and 4%(w/v)) showed that no effect to the pH, total sugars (as invert sugar), TPC and LAB (p>0.05). The fermentation at 30 ํC had %total acidity (as lactic acid), TPC and LAB higher than the fermentation at 20 ํC. But pH and total sugars (as invert sugar) decreased when fermented at 30 ํC. Lactobacillus plantarum, Lactobacillus brevis, Pediococcus spp. And Leuconostoc mesenteroides. Lactobacillus plantarum were detected in all fermentations. The results of sensory evaluation indicated that the fermented leaf mustard in 4% salt, 4% sugar at 30 ํC was more acceptable in aroma, texture and taste. The flavor analysis using gas chromatography/mass spectrometry showed that dimethyl disulfide, allyl isothiocyanate, benzaldehyde and dimethyl trisulfide were found as major volatile compounds in fermented leaf mustard.