The density fluctuations of supercritical CO2 and CF3H were measured for various thermodynamic states by small-angle X-ray scattering and also calculated using the empirical state equations. For each sample, the ridge of the density fluctuations forms along the extension of the coexistence curve of gas and liquid in the P-T phase diagram and seems to be the locus of a higher-order phase transition. In the rho-T diagram, the contour maps of the density fluctuations of the two samples an similar and the two ridges are indistinguishable from each other. The density fluctuation seems to be one of the very important and universal variables in describing the state of supercritical fluids.