Intensity oscillation of reflection high-energy electron diffraction (RHEED) is observed during C 60 layer epitaxial growth on GaAs (111)B and (111)A substrates. The frequency of the oscillation coincides well with growth rate of C 60 layers, suggesting that C 60 layers grow with repeating nucleation and a step flow growth as with GaAs and other semiconductor materials. Unusual oscillation is observed in the initial C 60 layer growth on GaAs (111)B substrates with (2×2) reconstruction. The initial layer growth is completed at approximately half monolayer coverage by C 60 molecules. This phenomenon is explained by the model that C 60 absorption sites are limited due to As-trimers absorbed on (111)B surfaces. This model is strongly supported by the fact that no such effect is observed on GaAs (111)A substrates where no As-trimer is absorbed.