Surface morphology of hillocks on the surface of various orientations of a GaAs substrate was observed by an atomic force microscope (AFM). Only (1 1 1)A and (2 1 1)A substrates formed pyramidal hillocks. The surfaces were the top layers of seven-period asymmetric double quantum wells or the buffer layer, and the Ga flux: and substrate temperature were kept constant at 0.76 M Lis and 520 degreesC, respectively. With increasing As pressure from 7.6 ML/s (beam equivalent pressure, BEP; 9.0 x 10(-6) Torr) to 32 ML/s (BPE; 4.5 x 10(-5) Torr), the hillock heights and slopes on (1 1 I)A first increased and then decreased at the highest pressures. The decrease in slope was large: at an As pressure of 1.95 x 10(-5) Torr. the hillock slope on ( 1 1 1)A was about 0.8 degrees, whereas at 2.85 x 10(-5) Torr, the slopes were usually less than 0.3 degrees. This reduction of the hillock height and slope at higher As pressure is explained by the decreasing of the super-saturation of GaAs species in a quasi-liquid layer, QLL of As due to increment of Ga desorption.