The formation potentials of trichloronitromethane (TCNMFPs) of four natural amino acids (glycine, threonine, lysine and tryptophane) were investigated during chlorination and preozonation-chlorination. The effects of initial ozone dosage, reaction time, pH, and nitrite concentration on the TCNMFP of selected amino acid (AA) solutions were carried out in detail. During preozonation-chlorination, TCNMFPs of glycine- and lysine-containing solutions were more significant than that containing the other two AAs. Increasing the initial ozone concentration resulted in a considerable increase of TCNMFPs from glycine, lysine, and tryptophane. The concentration of trichloronitromethane (TCNM) of glycine, threonine, and lysine solutions increased during preozonation-chlorination with the ozonation time extended. The effect of pH on TCNMFPs of amino acid by preozonation was compound-specific. In addition, the presence of nitrite ions contributes greatly to TCNMFP for all four types of amino acids. However, as the concentration of NO2- increased to higher levels, there was no significant further enhancement of TCNMFP. For chlorination alone, TCNM formed in all four amino acid solutions, with tryptophane solution having the highest TCNMFP.