For the the size classification of fly ash, the performance of a special type of cyclone classifier equipped with guide vane was investigated. Due to the wide variations in the size of fly ash, it is commonly practiced to use more than techniques to measure the entire size range. Since the size definitions vary on the basis of the measurement methods, it is necessary to convert one type of data to the other. Using a recently developed technique for inter-conversion of sizes, the separation efficiency of the cyclone classifier with guide vane was analyzed by adopting the Tromp curve. The results showed that the cut size of cyclone classifier was about 56 μm and the amount of bypass was 16%, which indicated an inefficient separation. This could be attributed to the heterogeneous nature of the fly ash particles, which limited sharp separation of the particles in the range of 10 ~ 50 μm. It was also found that the removal efficiency of unburned carbon from the raw fly ash was about 74.5% with a yield of 51%.