This thesis research was designed to evaluate the effects of maturity stages, cooking methods, and growing conditions on the phytochemical profile and antioxidant ability of Brassica vegetables. First, kale was used as a model vegetable species to evaluate the influence of maturity stages including microgreen, baby green, and mature vegetables. In the second study, mature kale was used to examine the effects of cooking methods on availability of the selected bioactive factors and the antioxidant properties. In addition, the effects of growing conditions on the health beneficial factors were evaluated using kale, broccoli, and red cabbage microgreens. The two growing conditions were chamber and windowsill, reflecting a commercial and a home growing condition. All of the testing vegetable samples were extracted with 70% methanol and evaluated for their phytochemical compositions, total phenolic contents, ABTS• scavenging capacities, and total dietary fiber. Five, twenty-three, twenty-six, and twenty-three polyphenols were tentatively identified in the kale microgreen, baby kale, mature kale harvested at 56th day, and mature kale harvested at 87th day using a HPLC-MS/MS method, with trisinapoyl-diglucoside, quercetin-3-sinapoly-diglucoside-7diglucoside, feruloyl-glucoside, and disinapoyl-diglucoside being the primary components in each, respectively. The aliphatic glucosinolates including sinigrin and glucoraphanin were predominant in the kale, broccoli, and red cabbage microgreens grown under both chamber and windowsill conditions. In addition, microwaved mature kale had a greater level of phytochemicals than that boiled. It was noted that the baby kale extract possessed the greatest ABTS• scavenging capacity. The results of this thesis may be used to improve vegetable consumption.