Effects of seawater treated by ozone on reproduction, chlorophyl-l a content, DNA and RNA content in Chlorella sp. were studied. Meanwhile, NH+ 4-N, NO-2 -N and NO-3 -N content in the seawater were analyzed. The results showed as follow s: 1. A certain concentrat ion of ozone treatment could increase reproduction and chlorophyl-l a content, but higher concentration of ozone treatment could inhibit repraduction of Chlorella sp. . 2. DNA content showed relative stability, RNA content was more sensitive to ozone treatment than DNA content, and ozone treatment could increase RNA content. 3. NH 4-N and NO- 2-N content in seawater by ozone treated decreased, while NO- 3-N content increased compared with that in control seawater.