The widespread occurrence of tetracycline (TC) in the aquatic environment poses a potential risk to aquatic ecosystem and human health. In this study, elimination of TC by photolysis and TiO2 photocatalysis were investigated by using mercury-free UVA-LED as an alternative light source. Particular emphasis was given to the effect of humic acid (HA) on the reaction kinetics and mechanisms of TC removal. Photolytic degradation of TC was slightly enhanced by HA due to its photosensitization effect, as evidenced by the increased steady-state concentrations of (OH)-O-center dot. The most abundant transformation product of TC, which was formed by the attack of (OH)-O-center dot radical, was enhanced during photolytic degradation. During photocatalytic experiments, HA dramatically inhibited TC loss due to the surface deactivation of TiO2 and (OH)-O-center dot quenching. The steady-state concentration of (OH)-O-center dot was dramatically, decreased in the presence of HA. Identification of transformation products showed that HA could inhibit the oxidation pathways initiated by (OH)-O-center dot during photocatalysis of TC. These findings provide further insights into the assessment of photolysis and photocatalysis for antibiotics elimination in natural waters where HA exists ubiquitously. (C) 2016 Elsevier B.V. All rights reserved.