Mobile armoring phenomena, which is the sorting of surface sand on an equilibrium river bed with gravel and sand mixture, is discussed through experiments using a steep slope channel and sand mixture, with widely distributed grain size. Ripples and dunes are hard to be formed and the bed is usually flat in channels of sand mixture with gravels. And the flat bed forms can be estimated in the category of transition by the Garde-Raju's diagram, if the mean diameter of original sand mixture is used as a representative diameter. Bed surface sand becomes coarser than the original sand mixture even if all the particles on bed surface are moving and the sediment discharge is equilibrium. method for the estimation of sand composition of bed surface is also, proposed and proved to be useful.