In this study, the impact of alpha-iron oxide (alpha-Fe2O3, 20-40 nm) and gamma iron oxide (gamma-Fe2O3, 20-40 nm) nanoparticles (NPs) on phytoplankton species Selenastrum capricornutum and Nannochloropsis oculata was investigated Characterizations of the NPs were systematically carried out by TEM, dynamic light scattering, zeta potential, X-ray diffraction, SEM, and Fourier transformation infrared spectroscopy. Acute toxicity was tested between 0.2 and 50 mg/L for each NP for a period of 72 hours exposure. gamma-Fe2O3 NP inhibited development of N oculata at the rate of 54% in 0.2 mg/L group with a high mortality rate of up to 82%. alpha-Fe2O3 NPs were less toxic that induced 97% mortality on N oculata at 10 mg/L suspensions. In contrast, alpha-Fe2O3 NP inhibited growth of S capricornutum strongly (73%) in 0.2 mg/L group. gamma-Fe2O3 NPs showed similar growth inhibition (72%) on S capricornutum in 10 mg/L suspensions. Despite the differential effects, the results indicated acute toxicity of alpha-Fe2O3 and gamma-Fe2O3 NPs on N oculata and S capricornutum.