Abstract: Organophosphate flame retardants (OPFRs) are a class of typical emerging contaminants with significant toxicity to marine microalgae. In this study, the effects of growth and antioxidant characteristics of Karenia mikimotoi under exposure to different concentrations (0.5, 5, 50, 500 and 5000 μg/L) of two typical OPFRs tris(2-chloroethyl)phosphate (TCEP) and triphenyl phosphate (TPhP) were studied. The results showed that the growth of K. mikimotoi were significantly inhibited by TCEP and TPhP. The average inhibitory rates of TCEP and TPhP on growth of algal cells on the sixth day of exposure were 36.39% and 36.69%, respectively. The soluble protein content and activity of superoxide dismutase (SOD) and catalase (CAT) both increased first and then decreased with the increased exposure concentrations of TCEP and TPhP. Furthermore, the contents of soluble protein and the activity of SOD in algal cells showed higher sensitivity to the stress of TCEP, while the activity of CAT was more sensitive to the stress of TPhP. The content of malondialdehyde (MDA) in algal cells increased with the increase of the TCEP and TPhP concentrations in a dose-dependent manner. And the content of MDA in algal cells treated with 500 μg/L and 5000 μg/L TCEP concentrations was significantly higher than that in the control (P<0.05), while similar results were found for TPhP only at 5000 μg/L (P<0.05). The results can provide a theoretical basis for evaluating the ecological effects of OPFRs.