It is known that rotten parts of orange fruit fluoresce when they are excited by UV light. From an analysis of orange fruit skin, it was found that one of the major fluorescent substances was flavonoid methyl ether group whose exciting wavelength was 365nm and fluorescence wavelength was about 530nm. Based on the result, a lighting device with 120365nm-wavelength LEDs was manufactured and fluorescence images of rotten citrus fruit were acquired by a machine vision system with the three lighting devices. In this paper, a machine vision system with a pair of white and UV LED lighting devices and a color TV camera was proposed for the orange fruit grading system which could automatically detect rotten fruits.