Abstract: N-substitutedphthalimide derivatives were studied by electrospray ionization massspectrometry (ESI-MS) in positive mode in conjunction with high-resolutionquadrupole time-of-flight mass spectrometry (Q-TOF-MS)， in order to study its’ fragmetation pathways. Based on mass spectrumbehaviors of the fragmentation pathways, all the five compounds showed asimilar cleavage progress. Classical losses of CO and methanol owing to thepresence of ester were observed in all these compounds; in addition, the lossof mother nucleus N-hydroxyphthalimide was observed in the five compounds. The characteristic ions ofcompounds 1—4were the ions which lost methanol and N-hydroxyphthalimide. However, the characteristic ion of compound 5 was the ion oflosing N-hydroxy phthalimide, whosesstructure was similar to tropylium ion, then continuously lost a molecular C₂H₂ after losing methanol and CO in a row.