The optical constants of superthin films have been obtained from the reflectance vs. angle of incidence measurements using synchrotron radiation in the 60-900eV soft X-ray region. Nonlinear least square curve fitting method is used for analyzing the measured data. Eesults are given for Samples of C, Au, and Pt prepared by ion-beam sputtering (IBS), and the film thickness, the rms roughness of both films and substrates are also determined.