Abstract： Objective To study the frequency of the derivative chromosome 9 [der(9)] deletion among patients with chronic myeloid leukemia (CML), and explore the application value of local-specific inspector (LSI) 9q34 probe in detect the der (9) deletion. Methods Cytogenetic analysis was performed by 24 h unstimulated culture, GTG banding and karyotype. Dual colour/dual fusion or extra signal(ES) DNA probe was used to perform interphase-FISH for the detection of bcr-abl fusion gene in 52 patients with CML. LSI 9q34 DNA probe was used to detect der(9) deletion. Results FISH results showed bcr-abl fusion gene existed in all 52 patients with CML. der(9) deletion was detected in 12 patients with ES/DCDF probe, but only in 11 patients with LSI 9q34 probe. Conclusion It's more efficient in detection of der(9) deletion with LSI 9q34 probe. Deletion of der(9) might be a poor prognostic factor for CML patients, and LSI 9q34 probe should be used in all the CML patients with positive bcr-abl fusion gene.