Aims: The use of microalgae as source of natural antioxidants is under explored in Malaysia. Previous studies haveshown that microalgae contain minerals, polysaccharides, amino derivatives, carotenoids and phenolic compounds. Thisstudy aimed to determine total phenolic and flavonoid compounds and antioxidant activity when microalgae(Nannochloropsis oculata and Tetraselmis sp.) and cyanobacterium (Anabaena sp.) were subjected to abiotic stresses.Methodology and results: Treatment of sodium chloride (NaCI), sodium hypochlorite (NaOCI) and copper (Cu2+) weregiven when the cultures reached the exponential phase of growth and were collected at three different time points. Nontreatedcultures were used as controls. Total phenolic and flavonoid contents were determined using Folin-Ciocalteauphenol reagent and aluminium chloride colorimetric assays. Antioxidant activities were measured using 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity assay. Tetraselmis sp. exhibited the highest phenolic contentunder copper stress (10.35 ± 0.33 μg GAE/mg extract). Nannochloropsis oculata showed the highest total flavonoidcontent under copper stress (33.85±3.16 μg QE/mg extract). Anabaena sp. showed the highest radical scavengingactivity under NaOCI stress (96.42 ± 0.26%).Conclusion, significance and impact of study: This study showed that total phenolic, flavonoid and antioxidantactivities in treated cultures were high compared to non-treated cultures. These microorganisms could be utilized as asource of useful bioactive compounds while exploiting its abund