This study was aimed at investigating the cytotoxicity of nitrite on haemocytes of the black tiger shrimp, Penaeus monodon. Total haemocyte count (THC), reactive oxygen species (ROS) production, DNA damaged cell ratio and apoptotic cell ratio of shrimp were determined after exposure to different concentrations of nitrite-N (0, 1, 10 and 20mgL −1 ) for 0, 6, 12, 24 and 48h. There was no significant effect on the analytic indicator of shrimp exposed to 1mgL −1 nitrite-N. THC decreased after 48h exposure to 10mgL −1 nitrite-N, and after 24h exposure to 20mgL −1 nitrite-N. Production of ROS was induced after 48h exposure to 10mgL −1 nitrite-N, and after 12h exposure to 20mgL −1 nitrite-N. The percentage of DNA damaged cell increased in P. monodon following 48h exposure to 10mgL −1 nitrite-N, and 24h exposure to 20mgL −1 nitrite-N. Apoptotic cell ratio increased after 48h exposure to 10mgL −1 nitrite-N, and 12h exposure to 20mgL −1 nitrite-N. These results indicated that nitrite-N would induce overproduction of ROS, and then resulted in DNA damage and cell apoptosis, and subsequently caused THC reduction. This process of oxidative stress was one of the toxicity mechanisms of nitrite on shrimp.