The reaction mechanism of the breakpoint chlorination process has been investigated by the computational chemistry based on density functional theory (DFT) calculations. Although ammonia had been known to be converted to chloroamines (finally to NCl3 via NH2Cl and NHCl2) by successive chlorination in the presence of HOCl, our previous experimental study observed only NHCl2 species with no trace of NH2Cl and NCl3 before the breakpoint. While the calculated energetics support the formation of NHCl2 from ammonia via NH2Cl by successive chlorination as mentioned, a more favorable decomposition pathway of NHCl2 to N-2 and HCl has been identified rather than the further chlorination of NHCl2 to NCl3. The computational approach employed in this study has shown its general usefulness in explaining experimental results by tracking down the most probable reaction pathways.