Loss of N through volatilization of ammonia is the main limitation for agricultural use of urea. The aim of this study was to evaluate the volatilization of NH3 from urea granulated (Ug) or urea coated (Ur) with two oxidized charcoals produced by the oxidation of eucalyptus charcoal with 4.0 mol L-1 HNO3, followed by the post-treatment with NaOH (CHox) or without post-treatment with NaOH (CHox-OH). Ug and Ur were produced with 5, 10, 15 and 20% (m/m) of CHox or CHox-OH. The volatilization of NH3 was evaluated as 100 mg of N in the form of Ug or Ur applied to samples of a Hapludalf placed in a system with continuous and controlled air flow. The Ug with CHox-OH or with CHox was more effective in reducing the volatilization of NH3 than the Ur. It was estimated that the Ug with 16% of CHox or CHox-OH, with 39% N, reduced the volatilization by 40% in relation to urea without the additives. Results show that charcoal oxidized is an effective additive for reducing ammonia volatilization of urea.