Activated carbon fiber (ACF) was modified by sulphuric acid impregnation and employed as adsorbent for sulfanilamide (SA) from aquaculture wastewater. The structural and chemical properties of the modified ACF were characterized by scanning electron microscopy, Fourier Transform Infrared Spectroscopy, and the Brunauer-Emmett-Teller nitrogen adsorption method. Effects of various parameters, such as modification time (0–75 min), solution pH (3–9), reaction time (0–24 h), and temperature (25–45°C) on SA adsorption onto ACF were investigated. The equilibrium data result showed that the adsorption behavior agrees well with both Langmuir and Freundlich models. The maximum adsorption capacity was 209 mg/g. The adsorption behavior of SA on the modified ACF fitted well with the pseudo-second-order kinetics model. Thermodynamic parameters showed that the adsorption reaction was a feasible, spontaneous and exothermic physical reaction.