This research was conducted to evaluate glucosinolate profiles, vitamin C, total phenol, total flavonoid, and free sugar (glucose, fructose, and sucrose) content, fatty acid composition, and antioxidant activity in floret and leaf of six cauliflower and broccoli cultivars. The level of chemical constituents as well as antioxidants significantly varied among crop types, cultivars, and their different parts, in that phytochemicals such as glucosinolate were statistically higher in florets compared with leaves in both broccoli and cauliflower cultivars. In contrast, total flavonoid and free sugar were found at higher levels in the leaf parts. The Asia purple cultivar exhibited statistically higher vitamin C (649.7 mg·100 g−1), total phenol (1345.2 mg·GAE 100 g−1), and total flavonoid (632.7 mg·CE 100 g−1) contents and consequently had the highest antioxidant activity (1.12 mg·mL−1) in its florets, while Baeridom and Bridal had the highest total glucosinolate (9.66 µmol·g−1) and free sugar (318.6 mg·g−1) contents, respectively compared with other cultivars. Likewise, the major fatty acids were palmitic (23.52%–38.42%), linoleic (13.09%–18.97%), and linolenic (26.32%–51.80%) acids, which comprised the highest compositional ratio (more than 50%) of polyunsaturated fatty acids (PUFAs) in most cultivars. Among the antioxidants, total phenol exhibited the most significant positive correlation (r = 0.698 **) with antioxidant activity, followed by vitamin C (r = 0.522 **) and total flavonoid (r = 0.494 **), indicating their significant contributions to total antioxidant activity.