Traffic sign recognition system is an intelligent vision system to recognize traffic signs on the road. This paper describes a new approach on red color thresholding for traffic sign recognition system. To extract red color from an image, a thresholding method based on the CIE-RGB chromaticity diagram was proposed. A g-r histogram of the image was developed by subtracting the normalized red of RGB color from the normalized green one. A threshold value is selected automatically by analyzing the g-r histogram. To evaluate the proposed method, comparison to the other red color thresholding methods was made using tested images taken from real environments with varying illuminations and backgrounds. From the experiment, our proposed method resulted in the thresholding quality with average score of 0.630 and average execution time of 0.038 seconds which is the best performance among the others.