Very few experimental data on the mechanical behaviour of unsaturated soils exists, particularly on the collapse behaviour under general stress states, because of the technical difficulties and time-consuming nature of measuring suction and deformation. This paper presents the results of a series of controlled-suction triaxial tests on the collapse behaviour of an unsaturated compacted clay with different initial dry densities and suctions. The collapse behaviour here includes deformation characteristics, such as volume changes, and hydraulic characteristics, such as saturation changes. It is found that the wetting-caused collapse mainly depends on the mean net stress and the initial density, and that the volume decrease reaches a maximum when the specimen is under the initial yielding mean net stress. It is also found that the soil-water characteristic curve in terms of suction and degree of saturation shifts upwards with increasing specimen density. The soil-water characteristic curve of a compacted soil mainly depends upon the current density, not directly upon the stress state. In addition, experimental data show that the collapse occurs mainly in an intermediate range of suction levels, which are neither very high nor very low, and that the wetting-caused volume decrease is accompanied by an increase in the degree of saturation.Key words: unsaturated soil, density, triaxial test, suction, collapse, degree of saturation.