Abstract: This study takes Skeletonema costatum and Prorocentrum minimum as subjects to test the toxicity of crude and fuel oil water accommodated fractions (WAF) and their dispersed water accommodated fractions (DWAF). According to the result, low concentration crude WAF, crude DWAF, and fuel oil DWAF are benefiting to the growth of Skeletonema costatum. As for crude WAF, it shows the best growth promoting effect with the concentration of 0.3 mg/L. While crude DWAF are not able to be effective if its concentration is higher than 0.5 mg/L and maximum population growth rate can be reached at 0.1 mg/L. Moreover, in the acute toxicity test, the value 96 h-EC₅₀ of four different oil spill pollutants to this two microalgae are quite different, ranging from 0.07 mg/L to 30.77 mg/L. Fuel oil DWAF is the most poisonous one among them, which 96 h-EC₅₀ for Skeletonema costatum and Prorocentrum minimum are 0.45 mg/L and 0.07 mg/L respectively. While Prorocentrum minimum is the most insensitive for the toxicity of crude WAF with the highest 96 h-EC₅₀ of 30.77 mg/L.