Abstract: Mass behavior of electrospray ionization and fragmentation mechanism of 4 harmful textile dyestuffs (direct red 28, basic red 9, disperse blue 1 and disperse yellow 1) were discussed using electrospray ionization-tandem mass spectrometry(ESI-MS/MS). Experiments demonstrate that polar groups in molecule affect the ionization behavior of strong polar dyestuffs (direct red 28 and basic red 9). But for weak polar dyestuffs (disperse blue 1 and disperse yellow 1), the electron-withdrawing groups and electron-donating groups have great effects on the ionization behavior. Most characterized ions are even-electron ions (EE) due to soften ionization of ESI. The chromophore groups in the researched dyestuffs, such as azo, triphenylmethane, anthraquinone and nitrobenzene are found to cleavage in the mode of collision induced dissociation (CID). In ESI-MS/MS spectra, there are two main fragmentation pathways of a EE: one involves loss of an neutral molecule to yield another EE, and the other involves loss of an neutral radical (EE⁰_•) to yield an oddelectron ion(OE_•). Beside, the molecular mass (m/z) of both EE and OE_• are found to follow their corresponding nitrogen rules in accordance with the nitrogen atoms contained.