Abstract: Experimental results have revealed that lightweight aggregate(LWA)concrete represents platform flow plasticity under high compressive stress and its ultimate strength surface intersects with the hydrostatic stress axis at two different points,which is completely different from the normal concrete as the ultimate strength surface is open-ended.Consequently,the strength criterion for normal concrete does not apply to LWA concrete.In this paper,a multi-axial strength criterion for LWA concrete is presented based on the Unified Strength theory proposed by Yu,which considers the strength characteristics of LWA under high compression.With this theory,a simple expression for the strength criterion can be obtained just by testing several representative points.The formulation is simple and easily to apply in practice.