Objective To explore the effects of different dose rates of X-ray under the same dose on cell clonogenic formation in non-small-cell lung cancer cell line A549 in order to provide experimental basis for clinical radiotherapy plan. Methods The A549 cells were cultured at low density and irradiated with X-rays at dose of 4 Gy and selected dose rates of 1, 2, 4 and 6 Gy/min, respectively, from a linear accelerator. The 8th day after irradiation, the cells were fixed and stained with Giemsa solution, and colonies containing at least 50 cells were counted. The plating efficiency and surviving fraction were calculated. Results The clonogenic number in non-irradiated cells was 88.6±4.6. The numbers were significantly reduced in irradiated cells at dose rate 1, 2, 4 and 6 Gy/min (12.3±3.4, 9.0±0.8, 5.6±1.0, 11.5±1.7, respectively) than that in non-irradiated control cells (F=678.799, P<0.05). The plating efficiencies were decreased in irradiated cells, especially in 4 Gy/min irradiated cells, which was lower than that in any of the other three dose rate groups (P< 0.05). Conclusions Though at same radiation dose, cancer cells have different clonogenic formation efficiency when irradiation with X-ray at different dose rates. Thus, treatment with optimal dose rate may improve the radiotherapy efficacy.