To identify fruits on the tree and determine their locations are the key to harvest fruits by robot. The main methods to locate fruits on trees were compared with. As the low identification rate of common machine vision system, a new method to identify the citrus in a tree canopy by means of infrared thermal imaging was put forward. About 45 infrared thermal images of citrus on trees were captured from the citrus orchard. It was found that the different thermal distribution among citrus, leaves and branches was about 1℃, and these differences clearly appeared in the gray-level image, which could be easily used to segment the citrus from other parts in the image by using binary image at T=190. Sobel and mask operators were used to extract the edge of the image. The results indicate that it is possible to identify citrus on trees by using infrared thermal imaging, and it is much easier than the methods used before.