We compare the superlattice modulation, microstructure, and epitaxy of Tl2Ba2Ca2Cu3O10thin films grown on MgO [100] and SrTiO3 [100] substrates by dc diode sputtering. Films grown on MgO were found to be quite clean with thecaxis perpendicular to the substrate. However, no in‐plane orientational relationship to the substrate was found. Films grown on SrTiO3, on the other hand, showed very good epitaxy to the substrate despite the presence of second phases. Films grown on MgO also exhibited a longer coherence length of the superlattice modulation than those grown on SrTiO3under identical conditions.