The impact of planetary mill pretreatment on corn stover size, crystallinity, and enzyme digestibility was investigated based on the grinding ball properties in an effort to reduce energy consumption. Alumina, zirconia, and steel balls were used, of which steel balls had the highest specific gravity but the lowest surface roughness while alumina balls had the lowest specific gravity and the highest roughness. The planetary mill pretreatment effectively reduced the size and crystallinity of the corn stover for all the ball types. Steel balls showed the highest size reduction effect in short milling time such as a 20 min and alumina balls were effective at improving the surface area of the corn stover in a 60 min milling time. The use of alumina balls resulted in the highest enzyme digestibility with a glucose yield of 91.9% when the milling time was 60 min. (C) 2017 Elsevier Ltd. All rights reserved.