Abstract: The fractal structure of LPG diffusion fire edge was theoretically studied on the basis of image processing and fractal theory. Each flame image was divided into five zones in the axial direction and the variation of the fire edge fractal dimension was axially analyzed. The relationships of flame length, fractal dimension and overall equivalence ratio were also investigated by changing the oxygen flow rate. The results show that both the length and the fluctuation range of flame gradually decrease with the increasing of peroxy coefficient, while the fractal dimension from the nozzle to the end of the flame decreases first and then increases within small range.