This study was carried out to develop a vinegar by an onion juice. Onions are considered to be a promising source of the vinegar because these are rich in sugars, amino acids and various nutrients. An Acetobacter for an acetic acid fermentation was isolated and used from vinegars produced by industrial goods or from matured Kimchi. When supplemented with 2-8% ethanol into an onionic juice medium, the highest production of the acetic acid was observed at 9 days by addition of 4% ethanol. Optimum temperature and aeration for acetic acid production were exhibited at $30^{circ}C$ and 200 rpm, respectively. A flask containing larger air-contact surface region for fermentation was produced the more acetic acid than that of a test tube. Taken all these together, an optimum condition for the acetic acid fermentation was over 9 days at $30^{circ}C$, 200 rpm with 5% alcohol and 2% initial acidity. When fermented by the upper condition, the final product contains 5.2% total acidity and less than 1% ethanol. These are suitable for conditions of fruit vinegar notified by the Ministry of Commerce, Industry and Energy.