Abstract: The effects of sulfadimethoxine sodium salt (SMS) and tetracycline hydrochloride (TC) on cell density, photosynthetic pigment and maximum photochemical efficiency (Fv/Fm) were studied with four red tide algae species Scrippsiella trochoidea, Karenia mikimotoi, Phaeocystis globosa and Skeletonema costatum. Concentrations of SMS and TC were 7.5 mg/L～240 mg/L in toxic tests. The results indicated that the maximum inhibitory rates of SMS on growth of four algae species were among 5.47% and 36.26%, while those of TC were among 0 and 35.21%. The growth inhibitory effect of TC on P. globosa was not observed in tested concentrations. The concentrations of photosynthetic pigment were influenced by two antibiotics. Concentrations of chlorophyll a and chlorophyll b in all algal species decreased significantly, while concentrations of carotenoids increased dramatically exposure to antibiotics. The maximum photochemical efficiencies of four algae decreased significantly exposure to TC at 50 mg/L, while no significant differences were detected in SMS treatments. The increases in cell density exposure to TC were detected in all algal species, with the highest ratios ranging from 11.30% to 38.89%. The growth stimulating effect of TC on P. globosa was much higher than those on other species. The toxicity effects of SMS and TC on P. globosa were much lower than those on other three algal species. High concentrations of antibiotics in wastewaters of aquaculture ponds, hospitals or pharmaceutical factories could influence the growth of algal species. The ecological risks of harmful algal bloom affected by antibiotics should be concerned in further study.