Horizontal single-walled carbon nanotubes (SWNT) on MgO(110) and MgO(001) single-crystal substrates were synthesized in a temperature range of 830-910 degrees C by chemical vapor deposition using Co catalyst particles. SWNTs grow preferentially along the < 110 > directions of the two MgO surface planes. For synthesis at 830 degrees C, the distribution of the radial breathing mode (RBM) frequencies of SWNTs grown on MgO(110) is centered at 142 and 168 cm (1), attributed to semi-conducting SWNTs. For the same synthesis time, the RBM distribution of SWNTs grown on MgO(001) is significantly broader. This difference would be due to the larger surface energy of MgO(110) which reduces the probability of coalescence between Co particles.