The surface treatment and characterization of ultrahigh modulus polyethylene(UHMPE) fiber has been an important research subject. In this study, in order to investigate the oxygen plasma-treated UHMPE fiber according to the treatment time, diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy was used as an analysis tool. The UHMPE fiber in a plain fabric was used for the surface analysis. The KBr overlay technique was efficient for enhancing the characteristic peaks from the UHMPE fiber surface. However, the KBr overlay made the comparison of the spectra among the UHMPE fiber samples difficult due to the different KBr overlay amount of each sample. In order to compare the relative peak areas of the oxygen plasma-treated UHMPE fiber according to the treatment time, DRIFT analysis was performed without KBr overlay under constant fiber orientation to remove the fiber orientation effect. The spectral subtraction was useful for investigating the minute change of the UHMPE fiber after the oxygen plasma treatment under the constant fiber orientation. The peak at 1896 cm(-1) was assigned to the crystalline combination mode of the UHMPE fiber and remained almost the same after the oxygen plasma treatment. This peak was used as an internal standard peak for the spectral subtraction and the peak area normalization. The relative peak areas in the DRIFT spectra of the UHMPE fiber were compared according to the oxygen plasma treatment time. The C=O group increases and the C-O group decreases as the oxygen plasma treatment time increases. The DRIFT analysis data of the oxygen plasma-treated UHMPE fiber correlated well with the electron spectroscopy for chemical analysis data and DRIFT spectroscopy is known to be useful for investigating the UHMPE fiber surface.