Sulfur compounds in the diesel were selectively derived into methylsulfonium salts by reacting with iodomethane in the presence of silver tetrafluoroborate, and characterized by positive-ion electrospray ionization (ESI) fourier transform ion cyclotron resonance (FT-ICR MS). The conversion ratios and react selectivities of the methylation for various sulfur compounds were investigated by gas chromatograph coupled with pulse flame photometric detector (GC-PFPD). Result shows that the sulfur compounds in the diesel can react with iodomethane easily at room temperature, the most of sulfur compound derived into methylsulfonium salts;the homologue of benzothiophene get the higher conversion ratio and react selectivity than the homologue of dibenzothiophene (DBT). It is found that primarily sterically hindered alkylated DBT, for example, 4- or 4-, 6- DBT, is recalcitrant to be methylated. Other than benzothiophenes and dibenzothiophenes, one- and two-ring sulfides, as well as other sulfur compounds with a double bond equivalent (DBE) value ranged from 1 to 12 are identified in the diesel.