To explore the role of mitochondria in the malignant transformation of immortalized human bronchial epithelial cells (BEAS-2B), BEAS-2B cells were treated with α particles at different doses of 0.1 Gy, 0.25 Gy, 0.5 Gy and 1Gy. And the exposure rate was 0.223 Gy/min. After the second and fourth irradiation, the cells were cultured for 40 generations. Proliferative capacity of the cells was measured by a soft agar clone test, and changes of mitochondrial morphological structure were observed by a transmission electron microscope. Mitochondrial DNA (mtDNA) copy number was detected by Q-PCR, and alterations in ROS production and mitochondrial membrane potential were analyzed by fluorescene-activated cell sorting (FACS). After the irradiation of α particles at dose of 0.25 Gy, mitochondrial morphological structure altered significantly. Soft agar clone formation rate, mtDNA copy number and ROS production increase, while the mitochondrial membrane potential decleased with the irradiation times. The results indicated that these changes may be used as indexes reflecting the phenotype of cell transformation.Cited