Some changes of growth rate, pigment content, SOD act ivity , POD activity, CAT act ivity and ultrastructure of Haematococcus pluvialis exposed to different dosage ultraviolet radiation have been studied, the results showed that only little ultraviolet radiation is lethal for H . pluvialis, and more microalga be died with longer illumination time till all microalga died after 8 minutes exposure, while the survivors exposed shorter than 4 minutes have higher growth rate than the control group. There is more chlorophyll and carotenoid content in the cell with longer radiation time, and remarkable increase of astaxanthin content after being radiated between 4- 5 minutes has also been discovered. Contrary to the decrease of CAT activity, SOD activity has much increased to resist the ultraviolet radiation till both losing activity after 5 minutes exposure when some of their organelles were damaged inreversibly by ultraviolet radiat ion, such as chloroplast, plastosome and Golgi body, and POD activity has also much increased after 3- 5 minutes exposure.