PURPOSE: To obtain the title powder which has a low bulk density and an improved hygroscopicity and is excellent in the workability in kneading with various resins, hand and physical properties by treating the surface of hide powder with a polyoctenylene resin.

CONSTITUTION: The surface of hide powder is treated with a polyoctenylene resin, preferably one produced by polymerizing cyclooctene and having one double bond per C8 unit, a molecular weight of 10,000 or more, a trans-content of 50% or more, and a crystallinity of 10% or more. The treating methods include one in which a polyoctenylene resin and hide powder are melt-kneaded by use of the conventional various melting and kneading devices. The surface- treated hide powder is improved in the low bulk density and hygroscopicity which are the defects of the conventional hide powder and in the workability in kneading with various resins and a synthetic leather made from the resultant mixture is more excellent in both hand feeling and physical properties than those of the conventional ones.