Four different types of molten metal fire marks were studied through X-ray diffraction (XRD) and 3D image processing software (Image-pro). As a result, the microstructure, average grain size, phase composition and its distribution was obtained. The results showed that on the surface of molten metal fire marks there mainly existed cubic crystal of Cu2O, cellular crystals of Cu and columnar crystals of Cu. And in four different modes of fire, the microstructure and composition of molten metal marks was of significant difference, that is: (1) the average grain size of molten marks by a short circuit fire was about 3～5 μm, and its Cu2O was the lowest; melt marks by circuit overload had an average grain size similar to short-circuit, but their Cu2O content was about 30%; (2) melt marks by secondary short-circuit contained a certain number of larger diameter micro-hole, and their average grain size was about 30 μm; broadening and splitting provision of the diffraction peaks of Cu and Cu2O showed that their ablation was the deepest; (3) melting marks by fire showed the equiaxed maximum intensity of Cu2O diffraction peaks, and there was almost no micro-holes.