The chemical oxygen demand(COD) in wastewater can be determined by near-infrared spectra(NIRs),however it is strongly influenced by the water absorbance.It is then very important to choose proper pathlength for improving measurement sensitivity and thus minimize the measurement error. In this study,we presented a series of experiments to determine the optimum pathlength for COD determination using a Fourier transform near infrared instrument with transmission measurement.Sixty wastewater samples for this study were collected from hoggery.Partial least squares(PLS) models were developed based on the spectral data which had been collected by using the cell pathlength of 1mm, 2mm,5mm,and 10mm.The performances of the differents models were compared.The results show that the pathlength of 10mm developes the optimal performance in the spectral range 800-1350nm and that of 1mm or 2mm of pathlength can be obtained in the spectral range of 1350~2500nm.In addition,our results suggest that two pathlength combination can improve the reliability and accuracy of the model in the region of 800~2500nm.This method of two pathlength combination may also be applied to the quantitative analysis of complex components in aqueous solutions.