The ultrastructure of chloroplast and  quantified antioxidant compounds such as α-tocopherol, ascorbic acid and beta carotene, semi quantitatively by comparing the peak areas of HPLC chromatograms,  were affected considerably in response to exposure of Azolla caroliniana to UV-B radiation for 24 and 48 h. The results show that the most striking effect of the enhanced UV-B treatment on the ultrastructure of A. caroliniana was observed after 48 h as markedly widened chloroplast area occupied by large amount of plastoglobuli in stressed fronds. Prolonged UV-B exposure of A. caroliniana for 48 h resulted in a reduction in peaks area for for α-tocopherol, ascorbic acid and beta-carotene. The results show that changes in the chromatogram of non-enzymatic antioxidants were correlated with the duration of exposure to UV-B radiation. These antioxidant metabolites play a vital role in protecting A. caroliniana to a certain limit against UV-B radiation.