Although grain sorting is one of the key factors of sediment transport, a troublesome measurement of mixed-size sediment transport has been prevented us from understanding its mechanism. The interaction between different size grains affects the sediment transport rate and the resultant bottom topography change. Furthermore, recently, prediction and control of sediment-size distribution are required for the evaluation of habitat of marine creatures. In this study, the phase dependent characteristics of armor coat due to the vertical grading of mixed-size sediment in sheetflow regime is experimentally investigated under the oscillatory flow. The unsteady structure of armor coat and its effect on the sediment motion are experimentally investigated in detail for both of the developing and the fully-developed stages of armoring.