Tumour cells may be resistant to radiotherapy that results in unsuccessful cancer treatment in patients. The aim of this study was to evaluate the sensitizing effect of atorvastatin (ATV) on breast cancer (MDA-MB-231) and non-small cell lung cancer (A-549) cells following exposure to ionizing radiation (IR). These cells were treated with ATV and exposed to X-ray at dose 4 Gy. The radiosensitizing effects of ATV were evaluated by flow cytometry and anti-proliferation assays. The production of reactive oxygen species (ROS) was determined in irradiated and treated cells with ATV. The findings of this study showed that ATV increased the percentage of apoptotic cells in irradiated breast and lung cancer cells. ATV exhibited anti-proliferative effect on cancer cells and increased cell death induced by IR. ATV increased ROS production in irradiated cells. The present study demonstrates that ATV has radiosensitizing effect on breast and lung cancer cells through increasing apoptosis, ROS production and cell death induced by IR.