The ammonia nitrogen stripping process at open horizontal flow reactors was evaluated and is described in this paper. The ammonia came from landfill leachate. Superficial loads of 650 kgN-<inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00387-ilm02.gif"/>.ha-1.day-1 (phase 1), 750 kgN-<inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00387-ilm03.gif"/>.ha-1.day-1 (phase 2), and 850 kgN-<inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00387-ilm04.gif"/>.ha-1.day-1 (phase 3) were applied with a hydraulic retention time of 51.2, 64.9, and 55.6 days, respectively. The experimental system setup was designed, built, and monitored at EXTRABES (Estação Experimental de Tratamentos Biológicos de Esgotos Sanitários) and consisted of a series of four open horizontal flow reactors fed with in natura leachate. The study was based on a stripping process with no alkaline chemical species addition, nor air flow for ammonium gas drag, and aimed to reduce the landfill leachate ammoniacal nitrogen concentration. The studied process showed a removal efficiency of 99.0% in the first phase, 99.3% in the second, and 99.5% in the third. Regarding the organic matter expressed in terms of CODtotal, the removal efficiency was 69.20% in phase 1, 40.12% in phase 2, and 29.23% in phase 3. It was noted that ammonia removal efficiency was directly proportional to the applied superficial load and the carbonaceous material removal was proportional to the organic matter applied in influent, since in phase 1 the CODtotal concentration removal efficiency was 1.7 times greater than that of phase 2, and 2.3 times that of phase 3. Briefly, the ammonia nitrogen stripping process by open horizontal flow reactor is a promising technological alternative for ammonia nitrogen reduction, mainly at regions with available area and temperature greater than 25°C. It is promising because it has no costs in terms of chemical reagents to raise the pH, nor does it require electric costs.