Abstract: Selection of path length has a great impact on near infrared spectroscopy(NIR) spectroscopy analysis. A series of experiments to determine the optimum path length for chlorpyrifos in pesticide EC determination was presented using a near infrared instrument with transmission measurement. The samples with different contents of chlorpyrifos were made by adding some dimethylbenzene solvent. NIR transmission spectrum of chlorpyrifos samples were collected with different optical path lengths (1, 2, 4 and 10 mm). Calibration models were established using partial least square (PLS) regression. The results showed that when using 4 mm path length cuvette spectral acquisition,the performance of the model with r= 0.983 and RMSEP=0.383 is optimal. Prediction set samples were used for t-test and the result indicated that there was no significant difference between the actual values and prediction values of the model, which indicate that the model developed using 4 mm path length for collecting spectra was accurate and reliable.