This paper investigates the stabilization of expansive soil with hydroxy-aluminium (Al13) with Al/soil ratios of 0.10, 0.14, 0.18, 0.22 and 0.26 mmol/g. A series of laboratory tests were conducted to study the effects of Al13 on the mechanical properties of expansive soil, including Atterberg limits, grain size distribution, swell percent, swell pressure and unconsolidated-undrained (UU) triaxial strength. The results revealed that Al13 reduced the plasticity index, clay content and swelling potential and enhanced the shear strength of expansive soil. The minerals and micro-structural changes in the soil samples were also determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the stabilized soil showed a remarkable flocculated-agglomerated structure characterized by the densification of particle associations. The consistency of the changes in mechanical properties and micro-morphology indicated that expansive soil can be effectively improved with the use of Al13. Additionally, the optimum dosage of polynuclear hydroxy-Al (PHA) solution for stabilizing expansive soil was 0.18 mmol/g under the given properties of expansive soil and PHA utilized in this study.