RHEED intensity oscillations have been observed during the epitaxial growth of layered NbSe2, MoSe2 on GaAs(1 1 1) substrates. Monolayer- and bilayer- mode oscillations have been observed at different diffraction points. The existence of bilayer-mode oscillation is interpreted as showing the polytype of the grown films. The present report demonstrate the utility of RHEED oscillation for analyzing the structure of ultrathin films of layered materials. The epitaxial growth of TaSe2 on layered substrates is also reported.