We have examined microstructures of the growth interface in ultrathin YBa2Cu3O7 (YBCO) films grown on (100) SrTiO3 and (100) MgO substrates. On both substrates YBCO films are completely c-axis oriented, and the surface is flat within ±one unit cell thickness. On the SrTiO3 substrare the atomic stacking sequence is usually SrO-TiO2-BaO-CuO2-Y-CuO2 BaO-CuO. In contrast, on the MgO substrate various sequences appear, because there are many narrow steps on the substrate. On both substrates the influence of steps tends to disappear as the film grows up to two unit cell thickness. The surface structure of YBCO was the same on both substrates: the film is always terminated by the BaO-CuO2-Y-CuO2-BaO-CuO sequence. © 1993, Japan Society of Powder and Powder Metallurgy. All rights reserved.