Abstract: Simulated wastewater and the wastewater from a coke-making factory were used to investigate the crystallization and flotation mechanism of precipitates formed in the precipitation of ammonia nitrogen in wastewater. The results obtained by X ray diffraction show that the precipitates formed in simulated wastewater are non-crystalline, but the precipitates formed in coke-making wastewater are crystalline. Infrared spectrum results show that the pH value at which the flotation of precipitates is carried out influences the mechanism between precipitates and collectors used for flotation of precipitates. The collectors, dodecanoic acid and sodium oleate, physically adsorb on the surface of precipitates at pH 9, but chemical adsorption of the collectors takes place on the surface of precipitates at pH 11. Dodecanoic acid is the best collector for the precipitates formed in simulated wastewater but the best collector for the precipitates formed in cokemaking wastewater is sodium oleate because of different crystallization states in different wastewaters.