The efficiency of the air classification of fly ash was investigated. Due to the wide variations in the size consist of fly ash, it is commonly practiced to use more than techniques o measure the entire size range. Especially for fly ash, the sieving techniques are used to obtain information not only on the size distribution but also on the carbon distribution throughout the size. However, there is a substantial amount in the sub-sieve size range, and consequently, sub-sieve analyzer is used to obtain the information on the size distribution in the finer end. Since the size definitions differ depending on the measurement method, 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 a commercial classifier was analyzed using the Tromp curve. The results showed that the cut size was about 55㎛ and the bypass was 17%. Sharpness Index was calculated to be 0.25, which indicated an inefficient separation. This could be reasoned from the heterogeneous nature of the fly ash particles, which limited sharp separation of the particles in the range of 30∼100㎛.