The degradation of dimethyl phthalate (DMP), which is a pollutant of concern in the water environment, was carried out by catalytic ozonation with N-methyl hydroxylamine (N-HA) at different pH. The process and mechanism were investigated for the removal of DMP and ozone (O3) under various experimental conditions including pH, N-HA dosage, and with or without tert-butyl alcohol (TBA). The results indicated that the removal of DMP was more efficient in the O3/N-HA system than sole O3, and the efficiency improved with increasing N-HA dosage. The levels of N-HA protonation were 99.89%, 8.36%, and 0.06% at pH 3.0, 7.0, and 9.2. Protonated N-HA (N-CH3NH2OH+) enhanced the degradation of DMP from 6.93% to 100% at pH 3.0, and it decreased when pH was raised to 7.0 and 9.2 because of the lower level of N-HA protonation. The results of adding TBA confirmed that hydroxyl radicals (•OH) were the main active species involved in the removal of DMP during the ozonation process.