Abstract: FT-IR-spectrometry was applied to the identification and differentiation of gel pen ink of different catagories extracted from handwriting with the solvent mixture of acetonitrile and water mixed in the ratio of 60 to 40 by volume.A total of 122 kinds of ink of different categories were taken as testing samples,and based on their efficiency of extraction,the ink samples were divided into 2 groups,i.e.,the solubles (63 kinds) and the insolubles (59 kinds).Further identification of the ink samples was performed by the FT-IR-spectrometry through inspection of number of characteristic peaks,position of the peaks and ratio of peak heights obtained from the IR-spectra,and informatoins about constituents of the ink sample,including the solvents,surfactants,resins and colorants used in making the ink,were obtained.Various influential factors to the IR-spectrometry,i.e.the sample preparation by grinding with KBr and pressing into pellets,the quality of paper on which handwritings were made and the UV-radiation at different wavelengths and for different time,were also studied.