Color of fruits is an important index for the classification of fruits, which will affect consumers′ purchase desire. An algorithm based on RGB color model is studied to detect the red area of Fuji apple. The light compensation of image is used to reduce the effect of illumination change and nonuniformity of light source. Two parameters of R/G and G/B in RGB image are calculated, and the thresholds of these two parameters are obtained by using training samples to achieve accurate segmentation for apple and background. The excess red minus excess green threshold segmentation method is applied to detect and calculate the red region of apple surface. The experimental results show that the algorithm based on RGB color model is able to accurately detect the red area of the apple surface. The algorithm can meet the requirement of color grading detection of Fuji apples.