Small-angle X-ray scattering (SAXS) experiments for supercritical C 2 H 4 were carried out, to study its density fluctuation and correlation length, which are quantitative descriptions of the inhomogeneity of molecular distribution. Except for thermodynamic states near the critical point, the density fluctuation from SAXS experiments is in very good agreement with the one calculated from a new state equation. For investigation on the substance dependence of the inhomogeneity in supercritical states, the results were compared with our previous ones of CO 2 ,CF 3 H and H 2 O. The density fluctuation and correlation length for C 2 H 4 show fairly similar behaviors to those of supercritical CO 2 ,CF 3 H as a function of reduced density ρ r =ρ/ρ c and reduced temperature T r =T/T c . On the other hand, characteristics for inhomogeneity of supercritical H 2 O are different from those of others. Good correspondence of static inhomogeneity and dynamic one in supercritical C 2 H 4 is revealed, by comparing the fluctuation from SAXS with its correlation time from dynamic light scattering experiments.