With the aim of studying molecular size effect on structural fluctuation, small-angle x-ray scattering (SAXS) experiments for supercritical benzene were performed for various pressures along three isothermal conditions at T-r=T/T-c=1.02, 1.04, and 1.06. From analysis of the SAXS intensities, density fluctuation <(DeltaN)(2)>/ and correlation length xi were obtained, which are closely related to many physicochemical properties of supercritical fluids. In order to relate the correlation length with the density fluctuation, direct-correlation range R was also obtained, which is defined by xi(2)=R-2<(DeltaN)(2)>/. The values of R are linearly dependent on density, and its isotherms agree with each other in the wider temperature and density ranges than the previous reports on critical phenomena. With regard to the substance dependence of structural fluctuation, the correlation length of supercritical benzene shows larger values than those of CO2, CHF3, and C2H4, while the density fluctuation of the former exhibits the same trend as the latter. The correlation lengths normalized by respective molecular sizes are in good agreement among CO2, CHF3, and C2H4 and benzene, except for H2O. As a result, the normalization is necessary for the discussion on universality of the correlation length. (C) 2003 American Institute of Physics.