Although ordinary improved soils such as cement-treated soil or air-formed lightweight soil has relatively high compression strength, the improved soil show high rigidity and brittle behavior. In this study, composite improved soil with lightweight and deformability was developed. This geomaterial is made by mixing granular material obtained from crushed lightweight soil and cement-stabilized soil in slurry as filling material. The cone penetration and triaxial compression tests were performed on the composite stabilized soil. Mechanical properties of composite improved soil were compared with those of air-formed lightweight soil or sand. From the test results, it was confirmed that the composite improved soil has well trafficability at the early curing period and is possessed of both properties of cohesive and frictional materials. It is also indicated that the composite improved soil is a geomaterial with lightweight, frictional property and deformability.