To determine some bioactive compounds of Amaranthus blitoides in different phenological stages, an experiment was conducted in a randomized complete block design with three treatments and three replications. Three phenological stages, including eight-leaf, flowering and seed production stages, were considered three treatment of experiment, some phytochemical traits and phenolic acids, including quercetin, rutin, chlorogenic acid and caffeic acid, were measured. The highest total chlorophyll content (2.05 mg g-1) was observed at the seed production stage, while the lowest total chlorophyll content (0.94 mg g-1) was observed at the eight-leaf stage. The highest content of chlorophyll a and b was observed in the seed production stage, whereas the lowest amount of chlorophyll a and b was observed in the eight-leaf stage. The highest content of carotenoids was observed at the seed production stage. At different phenological stages, the evaluated traits had significant differences. The highest amount of total phenolic compounds, total flavonoids, total tannins, and antioxidant capacity were observed in the flowering stage. HPLC analysis of extracts showed that quercetin and rutin production increased during growth until the flowering stage and then decreased. Also, the biosynthesis of two phenolic acids, including chlorogenic acid and caffeic acid, was high at the eight-leaf stage and reduced after this phenological stage. In general, the highest amount of bioactive compounds and phenolic acids quercetin and rutin of this plant was observed in the flowering stage and the highest amount of plant pigments was observed in the seed production stage.