For the size classification of fly ash, the performance of a special type of modified cyclone classifier equipped with a guide vane was investigated. Because of the wide variations in the size of fly ash, it is a common practice to use more than one technique to measure the entire size range. Because the size definitions differ depending on the measurement methods, it is necessary to convert one type of data to another. Using a recently developed technique for inter-converting sizes, the separation efficiency of the modified cyclone classifier equipped with a guide vane was analyzed by adopting the Tromp curve. The results showed that the cut size of the cyclone classifier was ca. 42 μm and the amount of bypass was 12 %, which indicated an inefficient separation. The finding could be attributed to the heterogeneous nature of the fly ash particles, which limit the sharp separation of the particles in the range 30~100 μm. We also found that the removal efficiency of unburned carbon from the raw fly ash was ca. 57% at a yield of 75%.