Electron mobility anisotropy on thermally oxidized (110) silicon surfaces with very low surface states, and hence negligible ion scattering, is observed and compared with both the classical and quantum theories. The observed anisotropy at low temperatures and high surface electric fields is considerably higher than that predicted by the classical theory, indicating quantization in the surface channel. However, the observed anisotropy is smaller than that predicted by the simple quantum theory of the lowest two electric subbands in which surface roughness and interband phonon scattering are neglected. It is shown that surface roughness of 3 to 5 Å based on Ziman's model exists and the energy separation of the two subbands is smaller than the intervalley phonon energies so that these effects should be included in an improved quantum theory.