Abstract: In order to find out an automatic monitoring method of plant insect pests, leaves infected by the leafminer were studied. Using image processing and spectroscopy analysis technology, near-infrared spectral reflectance of leaves infected by the leafminer was measured; damaged degrees(DD) of leaves and the ratios of the dry and fresh leaf weights(DFLW) were calculated. The regression results between DFLW and the near-infrared spectral reflectance are as follows: R² value between cucumber leaves and the near-infrared spectral reflectance is 0.79 (at 1452 nm) and the tomatoes' is 0.70(at 1450 nm); the R² value between the cucumber's DD and the near-infrared spectral reflectance is beyond 0.81(at 1436～1466 nm), tomato's is beyond 0.69(at 1436～1468 nm), respectively. The experiment and analysis results show that the leaf damaged degrees can be reflected by the near-infrared spectral reflectance of leafminer-infected leaves.