Immature corn grains are becoming increasingly popular as a snack/vegetable, facilitating human consumption in some countries. Of particular interest were their nutritional and functional properties. Therefore, plants at stages M1 (74 DAS), M2 (86 DAS), M3 (98 DAS), and maturity stage (116 DAS), where DAS refers to days after seeding, were investigated for changes of the nutrients, phytochemicals, and antioxidant activity in corn grains during maturation. Results revealed that during maturation of corn grains, the content of reducing sugar and crude protein decreased while starch and total lipids increased (p < 0.05). Total carotenoids first decreased, then increased, and then decreased to minimum at maturity stage. Analysis of the main carotenoid compounds determined by HPLC showed that lutein first increased and then decreased (p < 0.05), whereas the reverse was found for beta-cryptoxanthin. The change in zeaxanthin was consistent with total carotenoids. Total phenolic content decreased; nevertheless, different phenolic fractions varied with various maturation stages. The antioxidant activity determined by DPPH and FRAP assay in total phenolic extracts decreased during maturation, which may explain that antioxidant activity can be attributed to soluble phenolic and total phenolic content.