Reflection high-energy electron diffraction (RHEED) rocking curves have been studied for the Si(111)-((3)1/2×(3)1/2) R30°–Ag structure as a function of varying degrees of surface roughness as generated by Ar+ ion bombardment. Under increased surface roughness, most peaks in the rocking curves showed a characteristic change represented by splitting, shifting, and variation in intensity. This characteristic change in the rocking curves is identified as a result of surface step generation accompanied by modulation of the surface potential barrier. This surface roughness effect well explains the behavior of RHEED intensity oscillations during epitaxial growth and identifies the origin of phase differences of different beams.