Interfacial roughness parameters and lattice strain of Si0.4Ge0.6films with varying thickness epitaxially grown on Si(100) were determined using the techniques of grazing‐incidence x‐ray scattering and diffraction. The roughness ofboththe buried interface and sample surface follows a similar power‐law scaling behavior with an exponent &bgr; around 0.71 for films below the critical thickness, and it undergoes a large change above the critical thickness. Observation of such a scaling law thus establishes a quantitative correlation between the interfacial roughness and lattice strain, and also allows the prediction of interfacial roughness as a function of film thickness of this compound. ©1995 American Institute of Physics.