Large scale structural evolution of supercritical carbon dioxide along the isotherm at 32 °C was investigated with small-angle neutron scattering. The maximum of the density fluctuation, the so-called 'ridge', was confirmed with Ornstein-Zernike analysis. To investigate the structural change in more detail, the molecular distribution over a large domain was determined with a newly developed reverse Monte Carlo method. From the molecular distribution obtained, the pair distribution function and cluster-size distribution can be calculated. With increasing density of carbon dioxide, the cluster size increases monotonically, whereas the pair distribution function for the range of sizes shorter than 10shows a monotonic decrease. From this result, itis suggested that the structural change along the isotherm is caused by the change of balance between growth of clusters and increase of the average density. © IOP Publishing Ltd.