Monochromatic x‐ray CT at two different energies provides information about electron density without ambiguity due to the beam hardening effect. This information makes the treatment planning for heavy‐ion radiotherapy more precise. We have started a feasibility study on the dual‐energy x‐ray CT by using synchrotron radiation. We developed a linear scanning CT system in order to evaluate what precision in the measurement was achieved. The experiments were carried out using monochromatic x‐rays of 40, 70 and 80 keV. Comparison of measured electron densities with the theoretical values proved that these values were in agreement in 0.9 &percent; on average. We have developed a 2D‐CT system with a two‐dimensional scintillator array to take images in a short time. At present, it has been proved that the electron density is measured in the precision of about 1 &percent; with the 2D‐CT system. Effective atomic numbers are obtained as well from the dual‐energy x‐ray CT. The CT images are reconstructed based on each of the electron density and the effective atomic number. They present different features of an object, and the contrasts in the images differ from that in a conventional CT image. The feature of the images may help distinguish more clearly tissues or organs with aid of different contrast from the conventional CT images. © 2004 American Institute of Physics