Lightweight treated soil method has recently been developed to reuse the dredged soils as artificial lightweight geomaterials, the density of which range 1.0～1.2 g/cm3, for coastal construction projects. In the two types of lightweight soils, a foamed treated soil (FTS) and a beads treated soil(BTS), the slurry of dredge soil is mixed with cement and air form, or cement and EPS beads(the diameters are 1～3 mm), respectively. The lightweight treated soil method have been applied in several seaport and coastal airport projects, however the sites were only at the level shallower than -3 m to avoid the density change due to the large waterpressure and the possible washout during underwater placing. In this study, the full scale field placing tests of lightweight soils were carried out at the site of 10m below the sea level in Kumamoto Port in the purpose of the investigation on the material properties of lightweight soils placed under deep water. Especially the change of density through the process of mixing, transportation, placing and hardening was examined in detail. It is found that the lightweight soil method can be applied under sea water of -10m, although the part of the volume of mixed foam or mixed EPS beads is swept away during the construction process. However the wet-density of lightweight soil measured 1 year after the construction was almost same as that of 28-day specimen, while the 1 year strength was 40 % larger than the 28-day strength.