Restoration work of northern library of Bayon in Angkor Thom, Cambodia, is described in terms of geotechnical aspects. The original foundationconsists of compacted sand of about 5m in height with 10m and 15m in width and length. Partial dismantling was perfomrmed and reconstructed by slacked lime mixed soil. The paper reports general aspect of the ground condition, dismantling process, and reconstruction of the foundation with mechanical properties of compacted slacked lime mixed soil.