Abstract: MS characteristics of monosulfuron-ester were analysized by comparison between reversed-phase HPLC/ESI/MS and HPLC/APCI/MS. Monosulfuron-ester gave positive-ion ESI/MS spectra with major + and +, +, + ion and negative-ion ESI-MS spectra with dominant - and - ions. Fragments and structurally informative spectra such as +, - and m/z of 110.31,136.24,168.18,244.19 were found in APCI/MS with + ion in positive-ion mode as predominant and - ion in negative-ion mode as preferential.Accordingly,negative-ion of ESI/MS and APCI/MS could be utilized to identify monosulfuron ester by its molecular ions; whereas positive-ion of APCI/MS could be used for its identification study by the succeeding cleavage of the specific ions in multi MS/MS environment.