Pressure-induced structural modifications in high-entropy alloys with varying Al content, AlxCoCrFeNi (x=0.1, 0.3, 0.75, 1.5), have been investigated at pressures up to ~50 GPa by synchrotron X-ray diffraction and transmission electron microscopy (TEM). In AlxCoCrFeNi compounds with x≧0.3, all of which exhibit initial pure fcc structures, the proportionality between the Al content and the transformation pressure is observed. This is attributed to the large size of Al atoms relative to those of the other constituent elements, which leads to more structural distortion in Al0.3CoCrFeNi and subsequently an increase in the formation energy of the stacking faults. High-resolution TEM results show the variation of the stacking sequence from ABCABC (fcc) to ABABAB (hcp) in Al0.1CoCrFeNi following exposure to high pressure. In Al0.75CoCrFeNi, which exhibits an initial dual-phase structure, the result again shows the transformation to an hcp phase despite its higher Al content, which might be due to the presence of the bcc phase that is more amenable to the pressure-induced phase modification. However, the trend of transformation inhibition by increasing Al content is again observed, with Al1.5CoCrFeNi retaining its initial structures up to the highest pressure achieved.