The importance of foundation design for tall buildings is increasing with the size of mats getting larger and thicker to safely support the very large amount of loads. In this paper, the effects of the geotechnical conditions on the thickness of mat foundations were studied. Three tall buildings, which have shear wall-frame interaction system and outrigger system with 50 and 75 stories have been analysed with 16 different soil conditions by combining four different allowable bearing stresses; 1,500 kN/$m^2$, 2,000 kN/$m^2$, 4,000 kN/$m^2$ and 8,000 kN/$m^2$. As a result of analysis, in cases assumed in this study, the thicknesses of mat foundations were considered to be the thicker on the geotechnical conditions that consist of more than two kinds of rocks than those on the geotechnical conditions with one allowable bearing stress.