The sequence analysis of peptides was performed by nano-electrospray ionization Fourier transform ion cyclotron resonance tandem mass spectrometry(Nano-ESI-FT-ICR-MSn) and several peptides were chosen as examples.With the aid of the collision induced dissociation(CID),FT-ICR provides not only precise mass/charge ratio,but also structure information of the selected peptides.The fragment ions were identified according to the observed molecular weights and peptide sequence was determined successfully.So Nano-ESI-FT-ICR-MSn is a useful tool for identification of the amino acid sequence of peptides with high confidence.Besides,a pathway for the dehydration of y ions without amino acids containing carboxylic acid under sustained off-resonance irradiation collision-induced dissociation(SORI-CID) condition was proposed.