MIRRORCLE‐6X, the unique portable synchrotrons, generates brilliant hard X‐rays. The purpose of this study is to investigate the biological effects of synchrotron radiation on cancer cells to identify its effectiveness. A549 human lung adenocarcinoma and DU145 human prostate adenocarcinoma cells were used. We examined biological effects on cancer cells by colony formation assay, micronucleus assay, WST‐1 method, and apoptosis detection of flow cytometry. Results of these assay revealed that the biological effects of X‐ray generated from MIRRORCLE‐6X on cancer cells are similar to those of 4 MV X‐ray from linear accelerator (linac). © 2004 American Institute of Physics