Onion is vegetable with good nutrition and its growth could stand different kinds condition. It’s medical properties attract researchers attention. The author characterized the preparations and the phenolic compounds from fresh onions (red, yellow, white and shallots) mild pickles in brine and syrup using ultra performance liquid chromatography coupled with mass spectrometer (UPLC-PDA-MS). The results are reasonable and analysis is acceptable. The test material was identified: quercetin, isorhamnetin and derivatives of these compounds. The total content of phenolic compounds in fresh onions ranged from 0.72 to 89.47 mg.100g-1 dry weight, mild pickles in brine from 5.39 to 0.31 mg.100g-1, syrup from 2.85 to 0.31 mg.l-1. Polyphenol content was highest in fresh onions, and much lower in the material subjected marinating process.