Due to the toxicity of synthetic antioxidants, the demand for alternative sources of antioxidants over the years has increased. The interest in such compounds is related to their importance in human health and food quality. Microalgae, a group of organisms with high morphological diversity and able to produce a wide variety of biochemical compounds are considered a promising, non expensive source of antioxidant compounds (Guedes et al., 2013a). This work reports the screening of several different species of microalgae for their antioxidant behavior using the 2,2’-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid radical cation (ABTS•+) spectrophotometric assay for antioxidant activity (Guedes et al., 2013b) to 50 microalgae from the Coimbra Collection of Algae (ACOI). This method is based on the discoloration of a radical solution expressed as a radical inhibition activity. Recorded values obtained varied between 0.19 and 12.31 mg.L1 equivalent ascorbic acid concentration. In order to confirm the results, the initial screening was followed by the application of the ABTS•+ assay and another common method for antioxidant activity namely that using 2,2-diphenyl-1-picrylhydrazyl (DPPH•) to 8 selected strains from the first 50 and belonging to different taxonomic groups. The DPPH• method consists on the decrease of absorbance of a solution containing the radical in the presence of antioxidants (Brand-Williams et al., 1994). The results obtained for these 8 strains ranged from 2 to 30 mg.L-1 equivalent ascorbic acid concentration for the ABTS•+ assay and from 7 to 13% inhibition of DPPH•. Both sets of results indicate an interesting antioxidant potential in microalgae belonging to the groups Eustigmatophyceae and Chlorophyceae. Tested species of these groups showed ABTS•+ values comparable to grape and raspberry ethanolic extracts, confirmed also by the DPPH• method.