The increasing use of carbon nanotubes has led to their synthesis and releasing into the environment, especially aquatic ecosystems. Therefore, it is important to study the effects of their ecotoxicity and their genotoxicity on algae as the first chain of the food web. In this study, the effects of single-walled carbon nanotubes with a diameter of 1-2 nm on green algae Chlorella vulgaris were analyzed. Treatments include: 0 (control), 12.5, 25, 50, 100, 150 mg/l were chosen to survey their effects on C. vulgaris at different hours. Cells number, chlorophyll a concentration and specific growth rate were measured. The results showed that cell density decreased, and there was significant difference between control and other treatments (p<0.05). The maximum and minimum number of cells in the control and 150 mg/l treatments were about 6.2×104 (after 48 hours) and 2× 104 cells/ml (After 72 hours) respectively. Increasing trend in chlorophyll a concentration was observed by treatment of 12.5 mg/l up to 96 after exposure.  After 72 hours, the maximum chlorophyll a concentration was recorded (0.175 μg/l). There was significant difference between control and other treatments (p<0.05). The specific growth rate at the treatment of 12.5 mg/l nanoparticles was the highest value equal to 0.055 after 96 hours, and decreasing trend of specific growth rate of control sample with other treatments was observed. According to DNA damage, treatmented cells were prepared by Alkaline single-cell gel electrophoresis method (comet assay), and stained with fluorescent dye. Microscopic images showed that damaged cells appear as comets composed of an intensively stained comet head and a tail of migrated DNA fragments. There was not sequence in untreated cells (control). Thus, single-walled carbon nanotubes induced DNA damage in algal cells.