To obtain surface-treated powder homogeneously dispersible in various matrices in the state of primary particles by treating the surface of powder with a specific ratio of a polypeptide and its derived compound.

The objective surface-treated powder is composed of (A) various powders (inorganic or organic powder such as talc, mica and silk powder) having an average unit particle diameter of 0.01-50 μm and (B) a polypeptide (preferably polypeptide derived from soybean, collagen or albumen) and its derived compound adsorbed to the surface of the component A at a ratio of 0.1-5.0 wt.%. The treated powder can be produced e.g. by adding the component A to a solution containing the selected component B dispersed in the solution and adsorbing the component B in the state of primary particle unit or the dispersion is directly sprayed to floating component A to effect the adsorption of the component B. The treated powder has affinity to matrices, is homogeneously dispersible in the state of primary particle unit and exhibits characteristic properties of the powder.