To use onion(Allium cepa L.) residue as raw materials of food products, contents and stability of quercetin-related substance in onion were investigated. The amount of quercetin-related substance of onion was richer in the inedible portion(317.99mg%) than the edible protion(4.10mg%). Total quercetin-related substances of juice and residue from fresh onions were 2.26mg% and 1.57mg%, respectively, but they were changed to 1.50mg% and 2.96mg% by heating at 8$0^{circ}C$ for 10min. Quercetin was found to be stable at 20$0^{circ}C$ for 60 min, but it was unstable to light. When it was illuminated with 30W, the content was decreased and reached to 80~85% of the original content after 48 hours. Quercetin was not affected by pH, but quercitrin and rutin were unstable below pH 5.