Antioxidant and anticancer effects of methanolic extracts from the flesh (WFME) and peel (WPME) of white onion, the flesh (YFME) and peel (YPME) of yellow onion, the flesh (RFME) and peel (RPME) of red onion were studied. The content of total phenolics in WFME, WPME, YPME, YFME, RPME, and RFME were $0.260{\pm}0.01$, $4.480{\pm}0.23$, $0.319{\pm}0.02$, $719.12{\pm}37.36$, $0.248{\pm}0.01$, and $806.21{\pm}26.38\;mg/g$, respectively. The quercetin content of WFME, WPME, YFME, YPME, RFME, and RPME were $12.56{\pm}0.19$, $3.57{\pm}0.14$, $15.24{\pm}0.65$, $755.29{\pm}22.24$, $5.70{\pm}0.23$, and $774.03{\pm}29.48\;mg$/100 g, respectively. Like total phenolics, the highest 2,2'-diphenyl-1-picrylhydrazyl (DPPH) scavenging activities were found in RPME. However, inhibitory effects on lipid oxidation of RPME were similar to those of WPME and YPME. In addition, inhibitory effect of WPME, YPME, and RPME for human breast cancer cell (MCF-7) growth were 78.43, 81.90, and 96.52% while those on human prostate cancer cell (LNcap) were 71.58, 77.93, and 98.47% at $100{\mu}g/mL$, respectively. Total phenolics, quercetin content, antioxidant, and anticancer activities exhibited significant variation among the 3 onion varieties in this experiment. Therefore, it is assumed that antioxidant and anticancer activities were affected by the total phenolics and quercetin level of onion.