Evaluation of total phenolic content (TPC), total flavonoid content (TFC) and antioxidant activity through various assays of weed plants, Avena fatua ( wild oat) and Chenopodium album (common lambsquarters), has never been reported before. In the present study, methanolic seed extracts of A. fatua and C. album were used. Total phenolic and flavonoid contents were determined by Folin-Ciocalteu and aluminum chloride methods, respectively. Moreover, antioxidant activity was determined through DPPH scavenging method, phosphomolybdenum assay and Ferric ion (Fe⁺³) reducing assay. The TPC values of methanolic extracts were 26.1 and 28.9 mg GAE/g dw while TFC values were 1.11 and 2.23 mg CE/g dw for A. fatua and C. album, respectively. The antioxidant activity of both extracts had shown 62% and 74% inhibition at 0.1 mg/mL extract in DPPH free radical scavenging assay. In addition to that the total antioxidant activity through phosphomolybdenum assay, the values of 75 and 120 mg AAE/g dw of both extracts were obtained. Ferric ion (Fe⁺³) reducing antioxidant power assay exhibited high antioxidant activity in both extracts by using ascorbic acid as a positive control. These results showed that phenolic and flavonoids were found in high concentration in two weeds which had strong antioxidant activity. However, C. album had higher antioxidant activity as compared to A. fatua.