Single-walled carbon nanotubes (SWNTs) with preferred 2D orientations were synthesized for the first time by chemical vapor deposition (CVD) of methane on silicon surfaces terminated with hydrogen, native oxide, and an ultrathin aluminum layer. The SWNTs grown on Si(100)-based surfaces take two perpendicular directions, and those grown on Si(111)-based surfaces take three preferred directions that are separated by 60 degrees, Simulations of SWNTs on Si(100) and Si(111) surfaces indicate that the observed orientation locking is the result of interactions between nanotubes and the surface lattices. The calculated orientations of energy minimums accord exactly with the experimental results and are independent of the tube chirality as observed. In addition, the calculations also show that the (111) surface is stickier than (100), thus explaining the observation that SWNTs grown on Si(111) surfaces are shorter than those on Si(100) surfaces.