Cluster-formation in I-propanol aqueous solution has been examined by a reverse Monte Carlo analysis of neutron small angle scattering intensity measured on the solution with an alcohol content of 0.167 in mole fraction. Cluster-size distribution, structure of cluster and composition of cluster have been analyzed. The cluster-size distribution ranges from very small value, as small as one propanol molecule, to larger one continuously. By the addition of salt the mean size of cluster increases, while the sizedistribution itself is scaled very well between the salt-free solution and salt-added solution, showing clearly a self-similarity of the mesoscale structure of both solutions. The ratio of the number of water molecules to 1-propanol molecules in each cluster tends to be about 2 at larger clusters regardless of salt content, giving some insight into a microscopic origin of cluster-formation of the current solution.