We investigated the effect of surface treatment in an ultrafiigh-vacuum chamber on the Q factor of ultrathin single-crystal silicon cantilevers with different thicknesses (60, 170, and 500 nm) and different surface orientations. When the length is L>30 mum, the Q factor is proportional to the thickness and surface loss dominates, whereas when L<30 m, support loss surpasses the surface loss. Heating can remove the SiO2 layer and absorbates and results in an increase of the Q factor. Hydrogen termination leads to a larger relative increase of the Q factor in thinner structures than in thicker ones. Heating and H exposure improve the Q values of Si(100)-oriented cantilevers more than Si(110)-oriented ones, and they result in the opposing resonance frequency response for these two surfaces.