Fruit picking robot is required for agricultural automation for fruit harvest, and vision system is theimportant and crucial composition of a robot system. An automatic extraction method of fruit objectunder complex agricultural background for vision system in fruit picking robot is presented in this study.The method is based on an improving Otsu threshold algorithm using a new feature in OHTA colorspace. Color features are extracted in OHTA color space and then used as an input for the Otsu thresholdalgorithm which calculates the segmentation threshold value automatically. Four kinds of fruit imagesare selected to validate the automatic extraction method. The fruit objects are automatically extractedwith this method and the outputs are presented in binary images. Numerous of experiments show thatthe automatic extraction method can extract mature fruit from complex agricultural background and theextraction accuracy is more than 95%. The results indicate an effective fruit object extraction method forvision system of fruit picking robot.