The condensation behavior of sulfur trioxide/sulfuric acid (SO3/H2SO4) in flue gas is closely related to corrosion, aerosol formation, and blue plume discharge. Thus, it has become a hot issue in energy and environmental fields. However, no accurate method is currently available to observe this condensation behavior. In this work, an experimental method was developed to explore the fate of SO3/H2SO4 in a temperature-decreasing flue gas flow among the range of 260 degrees C-40 degrees C. A novel sampling unit with quartz wool and sodium chloride (NaCl) layer in series was adopted to accomplish the task of collecting aerosol and gaseous H2SO4, respectively, from flue gas. Bench scale experiments were performed to verify the reliability of the method. A state diagram was created according to the experimental results to reveal the fate evolution of H2SO4 as the flue gas temperature dropped. On the basis of the state diagram, the existing empirical models for predicting acid dew point (ADP) were evaluated. The predicted ADP values were found to deviate from the definition, falling within the temperature range in which H2SO4 condensed rapidly instead of the point in which H2SO4 was beginning to condense. The state diagram derived in this work can reveal slight acid condensation behavior better than other methods could.