Use of foamed mixture lightweight soils has had significant impact on countermasures mitigating soft ground settlement, on earth pressure exerted on retaining walls, and in similar situations. This paper presents the effective utilization of coal ash as a geomaterial of foamed mixture lightweight soils produced in abundance from coal-fired power stations. We investigated effects of curing conditions(air, fresh water and seawater) on the strength and stiffness of foamed mixture lightweight soils utilizing coal ash by unconfined compression tests measuring stress-strain. We found that the strength and deformation characteristics are considerably influenced by curing conditions. The relationship between unconfined compressive strength and curing time was also expressed by the hyperbolic equation over a wide range of mix proportion conditions irrespective of the curing conditions. These properties show that seawater is more effective for curing foamed mixture lightweight soils than air and fresh water.