Abstract: According to the definition, the calculation principle and formula of the true reservoir thickness, the true thickness of reservoirs has been calculated with mathematics and geophysical methods based on the geological conditions. With the top and bottom depth of the reservoirs, dip angle and trending of structure, the formula for calculation of the true reservoir thickness has been derived. Comparing with the conventional method, it is not necessary to assume that top and bottom surface of the reservoirs are parallel, which is one of the biggest advantages in calculation of the true reservoir thickness. By making foil use of the bottom structure information, the true thickness of the reservoir can be calculated more accurately. This method is simple and accurate, with no errors.