It is difficult and high-cost to detect flame fronts by laser-sheet diagnostics under microgravity (micro-g), thus image processing is critical to obtain valuable information from the raw data. In the present study, premixed V-flames were detected under micro-g by OH planar laser-induced fluorescence (PLIF) and an effective method based on active contour model (ACM) is presented for automatic detecting and tracking flame fronts in the PLIF images. ACM can effectively detect the flame front in the images with low contrast and noises. Compared with other methods of flame front detection, the advantage of this method is that the image smoothing and image enhancement are not necessary for the correct detection of flame fronts in raw PLIF images.