Abstract: Luteinizing releasing hormone analog (LRH-A), renin substrate tetradecapeptide and angiotensin I were analyzed with infraredmatrix assisted laser desorption ionization/Fourier transform ion cyclotron resonance mass spectrometry (IR-MALDI/FTICR MS). 2,5-Dihydroxybenzoic (DHB) and succinic acid (SA) were used as matrices. The m/z of oligopeptides were determined with an error less than 4×10^-6. A series of characteristic fragment ions of peptides were obtained by increasing the laser power density appropriately. Their elemental compositions and the first order structures were confirmed. A sensitive, rapid and accurate method was established for the determination of oligopeptide.