This paper presents a new method for calculating vapour/liquid ratios of fluid inclusions based on highresolution petrographic microscopy and fluorescent microscopy. The microscope can offer a series of twodimensional images of single fluid inclusions by regulating its own dial. The following procedure is to draw the outline and measure the area of inclusion and vapour in each twodimensional image with drawingsoftware. The volume of inclusion and vapour bubble can then be determined by ① accumulating the slices which are described as irregular cylinders using equation, and ② integrating the pseudofunction of area and depth. The vapour/liquid ratio can then be calculated. The new method can provide vapour/liquid ratios of not only fluorescent hydrocarbonbearing fluid inclusions but also nonfluorescent aqueous fluid inclusions. Otherwise, it can also serve the insitu thermal variation analysis of fluid inclusions. The new method just requires ordinary instruments, so it costs little. Therefore, the method can be widely used and can help to develop the thermodynamic simulation of fluid inclusions.