In this study, sediment toxicity and bioavailability of copper and zinc to Phaeodactylum tricornutum were determined with and without sediment. Phaeodactylum tricornutum was exposed to different copper and zinc concentrations (250-2000 ng L-1) for 96 hours. A significant positive correlation between inhibition and metal concentrations and between the total cell counts and the chlorophyll-a content of Phaeodactylum tricornutum was observed. The metal concentration significantly (p<0.05) rose the inhibition values at the end of 96 h. The EC50 values indicated that it was more sensitive to copper and zinc with sediment than without sediment, and zinc was more toxic than copper both with and without sediment. The presence of sediment significantly (p<0.05) affected the inhibition values. The mean bioaccumulation and the mean bioconcentration factor rose with the increasing metal concentration, being with and without sediment. The metal concentration level significantly (p<0.05) affected the mean BA and BCF values. The exposure time significantly (p<0.05) affected bioavailability of metals. The sediment significantly (p<0.05) affected the mean BA values with copper, but did not significantly (p>0.05) affect the mean BA values with zinc. © by PSP.