This study was conducted to assess the removal of residual pesticides and to obtain high amounts of quercetin in onion peel extracts (OPEs) by 4 washing treatments. Washing is one of the standard processing steps in obtaining functional food ingredients from onion peel. After a first detergent wash (0.2% w/v) (DW) and hot air drying ($80^{circ}C$, 24 hr) (B), 4 washing treatments were tested, including a second DW (C), ultrasonication ($50^{circ}C$, 10 min) plus DW (D), 0.3% $H_2O_2$ (v/v) plus DW (E), and blanching ($95-97^{circ}C$, 2 min) plus DW (F). This was followed by 60% (v/v) ethanol extraction and vacuum freeze drying of the OPE. The E treatment yielded 89.04% OPE and a quercetin content of 96.84% in the OPE compared with the B treatment, and had the highest efficiency of all treatments tested. The OPE was tested for the presence of 177 residual pesticides and three compounds were detected in all treatments: cyhalothirn, fluquinconazole and procymidone. Cyhalothirn and fluquinconazole levels were below the permitted levels for fresh onion, while procymidone was present in the high level range of 128.01~133.46 mg/kg in all samples. The E treatment was a better washing method than the others for removal of residual pesticides. It could reduce the level of residual pesticides without changing the functional properties of the OPE.