Energy losses of 0.63-1.03 MeV/atom Sin (n ≤ 3) clusters in thin carbon films (2-12μg/cm2) have been measuredby using a special experimental set-up with the Rutherford backscattering (RBS) technique. The experimentalresults show that, for a givenenergy per atom, the energy loss of Si+2 ions, as well as Si+3 ions, is significantly larger than that of Si+ ions. This is the first observation of energy loss enhancement in measurements with theRBS technique. The enhancement in energy loss (I.e. The cluster effect) is evident enough when the effectivethickness of the carbon target is thin, but not evident when it is thick. The effect will not be obvious until the energy of the projectile is over a certain limit for a special target. This suggests that the cluster effect only occursin the first layers of the target film, and that the energy of the projectile must be large enough for the effect tobe seen.