The degradation of tetracycline over Fe3O4 catalyst was studied by using a coupled ultrasound/heterogeneous Fenton process. The effects of some key reaction parameters, the evolution of toxicity, and the reaction mechanism were investigated. Experimental results showed that the stability of catalyst was significantly improved when ultrasound was employed. Under optimal conditions, 93.6% of tetracycline was removed after 60 min of treatment. The removal efficiency of the total organic carbon (TOC) at 60 min was 31.8%. The surface hydroxyl radicals were identified as the major reactive species during the oxidation process. Toxicity tests with Daphnia magna indicated that the toxicity of the solution increased during the first 60 min and then decreased as the oxidation proceeded. (C) 2015 Elsevier B.V. All rights reserved.