Purslane (Portulaca oleracea L.) contains a variety of natural products with different biological properties. The objective of this research was to estimate changes in total phenolics, flavonoids, and fatty acids (α-linolenic acid and linoleic acid) contents as well as antioxidant activity of P. oleracea at different growth stages. Seeds were germinated in soil-filled plastic pots at greenhouse. Leaves and stems were collected at the vegetative and flowering stages. Total phenol and flavonoid contents of the samples were determined by Folin–Ciocalteau and aluminum chloride methods, respectively. The contents of α-linolenic and linoleic acids were determined using gas chromatography analysis after transesterification of fatty acids. Furthermore, Ferric reducing antioxidant power and 1,1-diphenyl-2-picrylhydrazyl assays were used to determine the antioxidant activities. The highest contents of total phenols (698.6 mg GAE 100 g⁻¹ DW), flavonoids (46.9 mg QE 100 g⁻¹ DW), α-linolenic acid (2.7 mg g⁻¹ DW) and linoleic acid (0.8 mg g⁻¹ DW) were determined in the leaves at flowering stage. Free radical scavenging capacity was significantly affected (P ≤ 0.05) by age; and the leaves of purslane had more antioxidant potential compared to stems. A positive correlation was observed between the antioxidant activities and total phenols content. Overall, purslane leaves at flowering stage can be regarded as a valuable source of fatty acids (especially α-linolenic acid) and antioxidants in human diet.