Abstract: In automatic detection of coal petrology by image analysis technology, it is found that the boundary of various components in the microscopic image of coal petrology demonstrates a "rim" of gray transition zone, and the pixel of this rim cannot reflect the true gray level of the components on both sides. In order to analyze its influence on the identification and detection of macerals, the characteristics and causes of the false boundary between various adjacent components were studied with analysis of microscopic images collected from a large number of coal samples in China. Generally, the false boundary is ring-shaped or strip-like rim, showing the gray ramp characteristics in-between the gray levels of adjacent components. The width of gray ramp of the false boundary is related to the combination of various components in microscopic images. False boundary develops by different heights of the relief due to different hardness and toughness of various components in the coal. The gray ramp appears at the boundary of different components during imaging. 10 representative coal samples from different rank in China were selected, and the edge detection of the sample false boundary was extracted by Prewitt operator. The pixel of the false boundary transition zone was 10%-27% of the image of total coal particle. Compared with the standard results of manual identification done by domestic authoritative experts in coal petrology field, the results show that the deviation of the vitrinite, inertinite and liptinite group with the removal of false boundary is much lower than that with false boundary, the former is closer to manual identification result.