The traditional sense of the flame size detection methods had the high detection error rate and easily influenced by sunlight conditions, and the flame region segmentation of images with complicated background still existed suspecting flame area. In order to improve the existing test methods， a method to detect the flame size puted forward in this paper ,based on RGB and HIS color space. This method combined two kinds of color space of flame characteristics, defining a new flame judgment constraint condition, and via the ratio between RGB color component values to eliminate light effects. By testing the images about different background, the results proved that the proposed method could effectively measure the flame size.