Urea fertilizers in agricultural operations usually tend to produce large amounts of ammonia due to hydrolysis, therefore contribute to the air pollution. The purpose of this study was to study the potential black liquor from pulp industry as urease inhibitor. Characterization of the black liquor was carried out by Spectrophotometer Fourier Transform Infra-Red (FTIR) and Thermal Gravimetric Analyzer (TGA) instruments. Meanwhile, the determination of ammonia levels was carried out using UV-spectrophotometer. The black liquor used in this process contains OH stretching, C=O stretching, an aromatic ring vibration, ring vibration, and guaiacil ring vibration indicating the presence of lignin. TGA primary weight loss in black liquor occurs above 200°C. The addition of urease enzymes to urea tends to increase the release of ammonia. Meanwhile, the results showed that black liquor could prevent the nitrogen loss of urea.