The aim of the present study was to explore the efficacy and mechanism of the radiosensitisation of icotinib hydrochloride (IH), a novel oral epidermal growth factor receptor-tyrosine kinase activity inhibitor, by evaluating the changes in tumour cell double-strand breaks (DSBs) repair, cell cycle and apoptosis following a combination of IH and radiotherapy (RT) in human colorectal adenocarcinoma cell lines. The HT29 and HCT116 human CRC cell lines were treated with IH and/or radiation. Effects on cell viability and cell cycle progression were measured by MTT, a clonogenic survival assay, and flow cytometry. Immunofluorescent staining and western blot analysis were applied to detect the expression of γ-H2AX and 53BP1 in the different treatment groups. Finally, the in vivo effect on the growth of CRC xenografts was assessed in athymic nude mice. IH inhibited the proliferation and enhanced the radiosensitivity in HT29 and HCT116 CRC cells lines. IH combined with radiation increased cell cycle arrest in the G2/M phase compared to the other treatments in the HT29 cell line (P<0.05). Similarly, cell cycle arrest occurred in the HCT116 cell line, although this increase did not result in significant differences in the RT group (P>0.05). IH combined with radiation significantly inhibited the expression of γ-H2AX and 53BP1 based on results of immunofluorescent staining and western blot analysis. In vivo, IH plus radiation significantly inhibited the tumour growth compared to either agent independently. In conclusion, IH significantly increased the radiosensitivity of HT29 and HCT116 cells in vitro and in vivo. Radiation combined with EGFR blockade inhibited tumour proliferation, increased apoptosis, prolonged G2/M arrest and significantly enhanced DNA injury in colorectal cancer. These data support the clinical trials of biologically targeted and conventional therapies in the treatment of cancer.